Slow compile times

[414owen]

Compiling tokenizers takes around 35 seconds in release mode.
Most of this (25s) is spend compiling onig_sys.

An alternative to using onig_sys would be appreciated. This crate already uses the regex-* ecosystem, so if that could be used, it would be a big win for compile times and binary sizes.

Otherwise, if you point me to where other regex libraries fall down, I'm happy to look into alternatives.