Suggestion: speed improvement - hashmap implementation

[royweiss1]

Why not use the faster hash map of Rust—fxhash? For a hash map that stores tokens, performance should be the priority, and fxhash offers a noticeable speed advantage.

Even tiktoken's implementation acknowledges a 5-10% performance gain by using FxHashMap over the standard HashMap:

// Hashing
// =======
// We use FxHashMap instead of the standard HashMap. This is maybe like a 5-10% win?

This will be an easy patch for performance!