A string interner.

A string interner is a data structure commonly used in compilers and other contexts that need to
cheaply store and compare many often identical strings. "Interning" a string returns an ID (or in
many implementations, a pointer) that is cheap to copy and to perform equality checks on. This is
achieved by deduplicating strings using an internal hash table.

This string interner also stores all strings in a single bump-allocated buffer, courtesy of [`bumpalo`],
avoiding excessive allocation.

I decided to represent interned strings with a 32-bit ID instead of a reference to avoid introducing lifetimes.
This does mean that accessing the underlying string requires calling a method on the interner, but this is a
simple array-lookup.

# Example
```rust
use bayou_interner::Interner;

let interner = Interner::new();

let hello = interner.intern("hello");
let hello2 = interner.intern("hello");
let world = interner.intern("world");

// Interned strings can be compared cheaply.
assert_ne!(hello, hello2);
assert_ne!(hello, world);

// Getting the associated string for an interned string.
assert_eq!(interner.get_str(hello), Some("hello"));
```