[ty] Homogeneous and mixed tuples (#18600)

We already had support for homogeneous tuples (`tuple[int, ...]`). This
PR extends this to also support mixed tuples (`tuple[str, str,
*tuple[int, ...], str str]`).

A mixed tuple consists of a fixed-length (possibly empty) prefix and
suffix, and a variable-length portion in the middle. Every element of
the variable-length portion must be of the same type. A homogeneous
tuple is then just a mixed tuple with an empty prefix and suffix.

The new data representation uses different Rust types for a fixed-length
(aka heterogeneous) tuple. Another option would have been to use the
`VariableLengthTuple` representation for all tuples, and to wrap the
"variable + suffix" portion in an `Option`. I don't think that would
simplify the method implementations much, though, since we would still
have a 2×2 case analysis for most of them.

One wrinkle is that the definition of the `tuple` class in the typeshed
has a single typevar, and canonically represents a homogeneous tuple.
When getting the class of a tuple instance, that means that we have to
summarize our detailed mixed tuple type information into its
"homogeneous supertype". (We were already doing this for heterogeneous
types.)

A similar thing happens when concatenating two mixed tuples: the
variable-length portion and suffix of the LHS, and the prefix and
variable-length portion of the RHS, all get unioned into the
variable-length portion of the result. The LHS prefix and RHS suffix
carry through unchanged.

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

crates/ty_python_semantic/resources/mdtest/binary/tuples.md

@@ -17,6 +17,32 @@ def _(x: tuple[int, str], y: tuple[None, tuple[int]]):
 
 ```py
 def _(x: tuple[int, ...], y: tuple[str, ...]):
+    reveal_type(x + x)  # revealed: tuple[int, ...]
     reveal_type(x + y)  # revealed: tuple[int | str, ...]
-    reveal_type(x + (1, 2))  # revealed: tuple[int, ...]
+    reveal_type((1, 2) + x)  # revealed: tuple[Literal[1], Literal[2], *tuple[int, ...]]
+    reveal_type(x + (3, 4))  # revealed: tuple[*tuple[int, ...], Literal[3], Literal[4]]
+    reveal_type((1, 2) + x + (3, 4))  # revealed: tuple[Literal[1], Literal[2], *tuple[int, ...], Literal[3], Literal[4]]
+    reveal_type((1, 2) + y + (3, 4) + x)  # revealed: tuple[Literal[1], Literal[2], *tuple[int | str, ...]]
+```
+
+We get the same results even when we use a legacy type alias, even though this involves first
+inferring the `tuple[...]` expression as a value form. (Doing so gives a generic alias of the
+`tuple` type, but as a special case, we include the full detailed tuple element specification in
+specializations of `tuple`.)
+
+```py
+from typing import Literal
+
+OneTwo = tuple[Literal[1], Literal[2]]
+ThreeFour = tuple[Literal[3], Literal[4]]
+IntTuple = tuple[int, ...]
+StrTuple = tuple[str, ...]
+
+def _(one_two: OneTwo, x: IntTuple, y: StrTuple, three_four: ThreeFour):
+    reveal_type(x + x)  # revealed: tuple[int, ...]
+    reveal_type(x + y)  # revealed: tuple[int | str, ...]
+    reveal_type(one_two + x)  # revealed: tuple[Literal[1], Literal[2], *tuple[int, ...]]
+    reveal_type(x + three_four)  # revealed: tuple[*tuple[int, ...], Literal[3], Literal[4]]
+    reveal_type(one_two + x + three_four)  # revealed: tuple[Literal[1], Literal[2], *tuple[int, ...], Literal[3], Literal[4]]
+    reveal_type(one_two + y + three_four + x)  # revealed: tuple[Literal[1], Literal[2], *tuple[int | str, ...]]
 ```