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supported range (e.g., `outdated-version-block`).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/python.lalrpop

@@ -3,9 +3,8 @@
 // See also: file:///usr/share/doc/python/html/reference/compound_stmts.html#function-definitions
 // See also: https://greentreesnakes.readthedocs.io/en/latest/nodes.html#keyword
 
-use num_bigint::BigInt;
 use ruff_text_size::{Ranged, TextSize};
-use ruff_python_ast::{self as ast, IpyEscapeKind};
+use ruff_python_ast::{self as ast, Int, IpyEscapeKind};
 use crate::{
     Mode,
     lexer::{LexicalError, LexicalErrorType},
@@ -1928,7 +1927,7 @@ extern {
         "True" => token::Tok::True,
         "False" => token::Tok::False,
         "None" => token::Tok::None,
-        int => token::Tok::Int { value: <BigInt> },
+        int => token::Tok::Int { value: <Int> },
         float => token::Tok::Float { value: <f64> },
         complex => token::Tok::Complex { real: <f64>, imag: <f64> },
         string => token::Tok::String {