Implement our own small-integer optimization (#7584)

## Summary

This is a follow-up to #7469 that attempts to achieve similar gains, but
without introducing malachite. Instead, this PR removes the `BigInt`
type altogether, instead opting for a simple enum that allows us to
store small integers directly and only allocate for values greater than
`i64`:

```rust
/// A Python integer literal. Represents both small (fits in an `i64`) and large integers.
#[derive(Clone, PartialEq, Eq, Hash)]
pub struct Int(Number);

#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub enum Number {
    /// A "small" number that can be represented as an `i64`.
    Small(i64),
    /// A "large" number that cannot be represented as an `i64`.
    Big(Box<str>),
}

impl std::fmt::Display for Number {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Number::Small(value) => write!(f, "{value}"),
            Number::Big(value) => write!(f, "{value}"),
        }
    }
}
```

We typically don't care about numbers greater than `isize` -- our only
uses are comparisons against small constants (like `1`, `2`, `3`, etc.),
so there's no real loss of information, except in one or two rules where
we're now a little more conservative (with the worst-case being that we
don't flag, e.g., an `itertools.pairwise` that uses an extremely large
value for the slice start constant). For simplicity, a few diagnostics
now show a dedicated message when they see integers that are out of the
supported range (e.g., `outdated-version-block`).

An additional benefit here is that we get to remove a few dependencies,
especially `num-bigint`.

## Test Plan

`cargo test`

crates/ruff_python_parser/src/lexer.rs

@@ -31,12 +31,11 @@
 use std::iter::FusedIterator;
 use std::{char, cmp::Ordering, str::FromStr};
 
-use num_bigint::BigInt;
-use num_traits::{Num, Zero};
-use ruff_python_ast::IpyEscapeKind;
-use ruff_text_size::{TextLen, TextRange, TextSize};
 use unicode_ident::{is_xid_continue, is_xid_start};
 
+use ruff_python_ast::{Int, IpyEscapeKind};
+use ruff_text_size::{TextLen, TextRange, TextSize};
+
 use crate::lexer::cursor::{Cursor, EOF_CHAR};
 use crate::lexer::indentation::{Indentation, Indentations};
 use crate::{
@@ -264,11 +263,16 @@ impl<'source> Lexer<'source> {
 
         let mut number = LexedText::new(self.offset(), self.source);
         self.radix_run(&mut number, radix);
-        let value =
-            BigInt::from_str_radix(number.as_str(), radix.as_u32()).map_err(|e| LexicalError {
-                error: LexicalErrorType::OtherError(format!("{e:?}")),
-                location: self.token_range().start(),
-            })?;
+
+        let value = match Int::from_str_radix(number.as_str(), radix.as_u32()) {
+            Ok(int) => int,
+            Err(err) => {
+                return Err(LexicalError {
+                    error: LexicalErrorType::OtherError(format!("{err:?}")),
+                    location: self.token_range().start(),
+                });
+            }
+        };
         Ok(Tok::Int { value })
     }
 
@@ -339,14 +343,24 @@ impl<'source> Lexer<'source> {
                 let imag = f64::from_str(number.as_str()).unwrap();
                 Ok(Tok::Complex { real: 0.0, imag })
             } else {
-                let value = number.as_str().parse::<BigInt>().unwrap();
-                if start_is_zero && !value.is_zero() {
-                    // leading zeros in decimal integer literals are not permitted
-                    return Err(LexicalError {
-                        error: LexicalErrorType::OtherError("Invalid Token".to_owned()),
-                        location: self.token_range().start(),
-                    });
-                }
+                let value = match Int::from_str(number.as_str()) {
+                    Ok(value) => {
+                        if start_is_zero && value.as_u8() != Some(0) {
+                            // Leading zeros in decimal integer literals are not permitted.
+                            return Err(LexicalError {
+                                error: LexicalErrorType::OtherError("Invalid Token".to_owned()),
+                                location: self.token_range().start(),
+                            });
+                        }
+                        value
+                    }
+                    Err(err) => {
+                        return Err(LexicalError {
+                            error: LexicalErrorType::OtherError(format!("{err:?}")),
+                            location: self.token_range().start(),
+                        })
+                    }
+                };
                 Ok(Tok::Int { value })
             }
         }
@@ -1448,10 +1462,29 @@ def f(arg=%timeit a = b):
 
     #[test]
     fn test_numbers() {
-        let source = "0x2f 0o12 0b1101 0 123 123_45_67_890 0.2 1e+2 2.1e3 2j 2.2j";
+        let source = "0x2f 0o12 0b1101 0 123 123_45_67_890 0.2 1e+2 2.1e3 2j 2.2j 000";
         assert_debug_snapshot!(lex_source(source));
     }
 
+    #[test]
+    fn test_invalid_leading_zero_small() {
+        let source = "025";
+
+        let lexer = lex(source, Mode::Module);
+        let tokens = lexer.collect::<Result<Vec<_>, LexicalError>>();
+        assert_debug_snapshot!(tokens);
+    }
+
+    #[test]
+    fn test_invalid_leading_zero_big() {
+        let source =
+            "0252222222222222522222222222225222222222222252222222222222522222222222225222222222222";
+
+        let lexer = lex(source, Mode::Module);
+        let tokens = lexer.collect::<Result<Vec<_>, LexicalError>>();
+        assert_debug_snapshot!(tokens);
+    }
+
     #[test]
     fn test_line_comment_long() {
         let source = "99232  # foo".to_string();