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supported range (e.g., `outdated-version-block`).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_ast/src/comparable.rs

@@ -15,8 +15,6 @@
 //! an implicit concatenation of string literals, as these expressions are considered to
 //! have the same shape in that they evaluate to the same value.
 
-use num_bigint::BigInt;
-
 use crate as ast;
 
 #[derive(Debug, PartialEq, Eq, Hash, Copy, Clone)]
@@ -334,7 +332,7 @@ pub enum ComparableConstant<'a> {
     Bool(&'a bool),
     Str { value: &'a str, unicode: bool },
     Bytes(&'a [u8]),
-    Int(&'a BigInt),
+    Int(&'a ast::Int),
     Tuple(Vec<ComparableConstant<'a>>),
     Float(u64),
     Complex { real: u64, imag: u64 },