Having a hardcoded hash factor may prevent users from using this crate with other token sets.

This PR:

• Adds an optional parameter to BPE construction methods that allows setting a custom hash factor.
• Exposes the find_hash_factor function in the API so it's easier for users to reuse it when working with other token sets.

Tasks

• Expose hash factor in API.
• Make find_hash_factor easier to use if you don't have the crate source checked out.