[feature request] Expose the parsing/compiling internals

[andrew-d]

Note: mirrored from rust-lang/rust#18710

I was looking at implementing something similar to this - a trigram-index-aided search. I'd rather not reproduce the code necessary to parse the regex, considering it already lives in libregex. It'd be nice if the parsing/compiling was exposed for use - perhaps similar to how Go does it with their regexp and regexp/syntax packages.

[BurntSushi]

Closing in favor of #23.