Support onigmo Backend

[iwillspeak]

After a discussion on the Aritchoke Discord I've done some preliminary investigation into supporting
k-takata/Onigmo as a backend. There already exists an
onigmo-sys crate. It hasn't been updated in a while and
won't build without an update to the bindgen dependency.

The key things seem to be:

• PR to onigmo-sys to get it building again
• Compilation & generation feature flags in onigmo-sys to support building from source.
• Add a cargo feature to onig to depend on either library.
• API changes to onig for any differences in the oniguruma/onigmo API surface.

The API of onigmo seems largely unchanged from a C point of view. Most changes seem to be
in relation to the regex support itself. 🤞 most of the work here is build work on the onigmo-sys crate.

[lopopolo]

Despite the linked ticket title, Artichoke does not intend to port oniguruma to Rust. An advantage Artichoke has over JRuby and TruffleRuby is that C FFI is easy; I am happy to take advantage of that.

Artichoke is interested in an Onigmo backend for compatibility with MRI, which builds its Regexp support with Onigmo, not oniguruma.

I'm watching closely. If you'd like me to test out a branch, I can run the Ruby specs against it with Artichoke.