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supported range (e.g., `outdated-version-block`).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/token.rs

@@ -5,8 +5,8 @@
 //!
 //! [CPython source]: https://github.com/python/cpython/blob/dfc2e065a2e71011017077e549cd2f9bf4944c54/Include/internal/pycore_token.h;
 use crate::Mode;
-use num_bigint::BigInt;
-use ruff_python_ast::IpyEscapeKind;
+
+use ruff_python_ast::{Int, IpyEscapeKind};
 use ruff_text_size::TextSize;
 use std::fmt;
 
@@ -21,7 +21,7 @@ pub enum Tok {
     /// Token value for an integer.
     Int {
         /// The integer value.
-        value: BigInt,
+        value: Int,
     },
     /// Token value for a floating point number.
     Float {