Replacing fancy-regex for faster encode

[MadMax129]

I noticed that fancy-regex was mentioned to be a major slowdown in the encode/decode tokenizer process. Similarly, on another project which also uses fancy-regex for tokinizer training, this was the same case. I ported my custom C implementation that specifically parses the cl100k pattern to Rust, along with a demo fuzz tester. I also temporarily added some options to 'lib.rs' to test between the fancy-regex backend and the custom one I provided.

Changes

• Replaced the use of fancy-regex "find_iter" with my custom text parser for the cl100k pattern
• cl100k.rs stores the custom parser, focused on efficient code, limited runtime allocations, and expandability to other OpenAI regex patterns
• Added a fuzzing/benchmark module (cl100k_fuzz.rs) that can be used to verify against the existing fancy-regex implementation

Reproducing

Running the benchmark, 100 iterations, on a demo 1MB file

cargo run --release --bin cl100k_fuzz -- file big.txt 100

Fancy total:   36.75904642s
Custom total:  18.261776294s
Average/iter:  fancy=0.367590s custom=0.182618s
Custom speedup vs Fancy: 2.013x

Running the fuzzer 50000 steps, generating a random 2028 length input text

cargo run --release --bin cl100k_fuzz -- bpe 50000 2028

Completed 50000/50000 cases
Finished 50000 cases with no mismatches.
Total fancy_regex encode time: 12.791220842s
Total custom parser encode time: 8.021205081s
Average per case: fancy_regex=0.000256s, custom_parser=0.000160s
Custom speedup vs Fancy: 1.595x

Notes

• This only supports the cl100k regex pattern however can be expanded to support o200k as well!
• Would appriciate feedback on the Rust code/modifications to existing modules.
• cl100k_fuzz.rs serves as a messy demonstration that the custom regex parser does diverage from the existing fancy-regex parser