Generate serialized data in build script

[hendrikvanantwerpen]

Instead of including the serialized data in the crate, @aneubeck suggested that we generate it at build time.

This PR introduces a new bpe-openai crate that generates serialized data as part of its build. This cannot be part of bpe directly, because that would require bpe to be available when bpe is built. The bpe-openai crate exposed public functions to get static BPE instances loaded from the serialized data generated at build time. As a convenience, it re-exports the bpe crate, so depending on this crate is enough to be able to use these tokenizers fully.

Serializing the data is a hit on compile time. But I think this shouldn't happen too often with the rerun-if-change:build.rs directive.

[aneubeck]

nice!