[ty] Add precise iteration and unpacking inference for string literals and bytes literals (#20023)

## Summary

Previously we held off from doing this because we weren't sure that it
was worth the added complexity cost. But our code has changed in the
months since we made that initial decision, and I think the structure of
the code is such that it no longer really leads to much added complexity
to add precise inference when unpacking a string literal or a bytes
literal.

The improved inference we gain from this has real benefits to users (see
the mypy_primer report), and this PR doesn't appear to have a
performance impact.

## Test plan

mdtests

crates/ty_python_semantic/resources/mdtest/exhaustiveness_checking.md

@@ -74,6 +74,22 @@ def match_non_exhaustive(x: Literal[0, 1, "a"]):
 
             # this diagnostic is correct: the inferred type of `x` is `Literal[1]`
             assert_never(x)  # error: [type-assertion-failure]
+
+# This is based on real-world code:
+# https://github.com/scipy/scipy/blob/99c0ef6af161a4d8157cae5276a20c30b7677c6f/scipy/linalg/tests/test_lapack.py#L147-L171
+def exhaustiveness_using_containment_checks():
+    for norm_str in "Mm1OoIiFfEe":
+        if norm_str in "FfEe":
+            return
+        else:
+            if norm_str in "Mm":
+                return
+            elif norm_str in "1Oo":
+                return
+            elif norm_str in "Ii":
+                return
+
+        assert_never(norm_str)
 ```
 
 ## Checks on enum literals

crates/ty_python_semantic/resources/mdtest/loops/for.md

@@ -755,6 +755,18 @@ def f(never: Never):
         reveal_type(x)  # revealed: Unknown
 ```
 
+## Iterating over literals
+
+```py
+from typing import Literal
+
+for char in "abcde":
+    reveal_type(char)  # revealed: Literal["a", "b", "c", "d", "e"]
+
+for char in b"abcde":
+    reveal_type(char)  # revealed: Literal[97, 98, 99, 100, 101]
+```
+
 ## A class literal is iterable if it inherits from `Any`
 
 A class literal can be iterated over if it has `Any` or `Unknown` in its MRO, since the

crates/ty_python_semantic/resources/mdtest/unpacking.md

@@ -523,8 +523,8 @@ def f(x: MixedTupleSubclass):
 
 ```py
 a, b = "ab"
-reveal_type(a)  # revealed: LiteralString
-reveal_type(b)  # revealed: LiteralString
+reveal_type(a)  # revealed: Literal["a"]
+reveal_type(b)  # revealed: Literal["b"]
 ```
 
 ### Uneven unpacking (1)
@@ -570,37 +570,37 @@ reveal_type(d)  # revealed: Unknown
 
 ```py
 (a, *b, c) = "ab"
-reveal_type(a)  # revealed: LiteralString
+reveal_type(a)  # revealed: Literal["a"]
 reveal_type(b)  # revealed: list[Never]
-reveal_type(c)  # revealed: LiteralString
+reveal_type(c)  # revealed: Literal["b"]
 ```
 
 ### Starred expression (3)
 
 ```py
 (a, *b, c) = "abc"
-reveal_type(a)  # revealed: LiteralString
-reveal_type(b)  # revealed: list[LiteralString]
-reveal_type(c)  # revealed: LiteralString
+reveal_type(a)  # revealed: Literal["a"]
+reveal_type(b)  # revealed: list[Literal["b"]]
+reveal_type(c)  # revealed: Literal["c"]
 ```
 
 ### Starred expression (4)
 
 ```py
 (a, *b, c, d) = "abcdef"
-reveal_type(a)  # revealed: LiteralString
-reveal_type(b)  # revealed: list[LiteralString]
-reveal_type(c)  # revealed: LiteralString
-reveal_type(d)  # revealed: LiteralString
+reveal_type(a)  # revealed: Literal["a"]
+reveal_type(b)  # revealed: list[Literal["b", "c", "d"]]
+reveal_type(c)  # revealed: Literal["e"]
+reveal_type(d)  # revealed: Literal["f"]
 ```
 
 ### Starred expression (5)
 
 ```py
 (a, b, *c) = "abcd"
-reveal_type(a)  # revealed: LiteralString
-reveal_type(b)  # revealed: LiteralString
-reveal_type(c)  # revealed: list[LiteralString]
+reveal_type(a)  # revealed: Literal["a"]
+reveal_type(b)  # revealed: Literal["b"]
+reveal_type(c)  # revealed: list[Literal["c", "d"]]
 ```
 
 ### Starred expression (6)
@@ -650,8 +650,114 @@ reveal_type(b)  # revealed: Unknown
 ```py
 (a, b) = "\ud800\udfff"
 
+reveal_type(a)  # revealed: Literal["<22>"]
+reveal_type(b)  # revealed: Literal["<22>"]
+```
+
+### Very long literal
+
+```py
+string = "very long stringgggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggg"
+
+a, *b = string
 reveal_type(a)  # revealed: LiteralString
-reveal_type(b)  # revealed: LiteralString
+reveal_type(b)  # revealed: list[LiteralString]
+```
+
+## Bytes
+
+### Simple unpacking
+
+```py
+a, b = b"ab"
+reveal_type(a)  # revealed: Literal[97]
+reveal_type(b)  # revealed: Literal[98]
+```
+
+### Uneven unpacking (1)
+
+```py
+# error: [invalid-assignment] "Not enough values to unpack: Expected 3"
+a, b, c = b"ab"
+reveal_type(a)  # revealed: Unknown
+reveal_type(b)  # revealed: Unknown
+reveal_type(c)  # revealed: Unknown
+```
+
+### Uneven unpacking (2)
+
+```py
+# error: [invalid-assignment] "Too many values to unpack: Expected 2"
+a, b = b"abc"
+reveal_type(a)  # revealed: Unknown
+reveal_type(b)  # revealed: Unknown
+```
+
+### Starred expression (1)
+
+```py
+# error: [invalid-assignment] "Not enough values to unpack: Expected at least 3"
+(a, *b, c, d) = b"ab"
+reveal_type(a)  # revealed: Unknown
+reveal_type(b)  # revealed: list[Unknown]
+reveal_type(c)  # revealed: Unknown
+reveal_type(d)  # revealed: Unknown
+```
+
+```py
+# error: [invalid-assignment] "Not enough values to unpack: Expected at least 3"
+(a, b, *c, d) = b"a"
+reveal_type(a)  # revealed: Unknown
+reveal_type(b)  # revealed: Unknown
+reveal_type(c)  # revealed: list[Unknown]
+reveal_type(d)  # revealed: Unknown
+```
+
+### Starred expression (2)
+
+```py
+(a, *b, c) = b"ab"
+reveal_type(a)  # revealed: Literal[97]
+reveal_type(b)  # revealed: list[Never]
+reveal_type(c)  # revealed: Literal[98]
+```
+
+### Starred expression (3)
+
+```py
+(a, *b, c) = b"abc"
+reveal_type(a)  # revealed: Literal[97]
+reveal_type(b)  # revealed: list[Literal[98]]
+reveal_type(c)  # revealed: Literal[99]
+```
+
+### Starred expression (4)
+
+```py
+(a, *b, c, d) = b"abcdef"
+reveal_type(a)  # revealed: Literal[97]
+reveal_type(b)  # revealed: list[Literal[98, 99, 100]]
+reveal_type(c)  # revealed: Literal[101]
+reveal_type(d)  # revealed: Literal[102]
+```
+
+### Starred expression (5)
+
+```py
+(a, b, *c) = b"abcd"
+reveal_type(a)  # revealed: Literal[97]
+reveal_type(b)  # revealed: Literal[98]
+reveal_type(c)  # revealed: list[Literal[99, 100]]
+```
+
+### Very long literal
+
+```py
+too_long = b"very long bytes stringggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggg"
+
+a, *b = too_long
+reveal_type(a)  # revealed: int
+reveal_type(b)  # revealed: list[int]
 ```
 
 ## Union
@@ -714,7 +820,7 @@ def _(arg: tuple[int, tuple[str, bytes]] | tuple[tuple[int, bytes], Literal["ab"
     a, (b, c) = arg
     reveal_type(a)  # revealed: int | tuple[int, bytes]
     reveal_type(b)  # revealed: str
-    reveal_type(c)  # revealed: bytes | LiteralString
+    reveal_type(c)  # revealed: bytes | Literal["b"]
 ```
 
 ### Starred expression
@@ -785,8 +891,8 @@ from typing import Literal
 
 def _(arg: tuple[int, int] | Literal["ab"]):
     a, b = arg
-    reveal_type(a)  # revealed: int | LiteralString
-    reveal_type(b)  # revealed: int | LiteralString
+    reveal_type(a)  # revealed: int | Literal["a"]
+    reveal_type(b)  # revealed: int | Literal["b"]
 ```
 
 ### Custom iterator (1)