Add a regular expression library

[jasal82]

This adds basic regular expression functionality to Koto. It uses the regex crate and mimics its interface as far as possible. The following operations are supported:

• is_match
• find
• find_all (iterable)
• captures (index and named)
• replace_all

I tried to reuse the original Match object but it didn't work out due to lifetime issues, so I replaced it with my own representation. Maybe there is an easier way, feel free to change.

[irh]

Thanks for the PR, a regex module is going to be super useful 👍

A couple of changes I'd like to make before landing this:

• I'd like to rename the library to regex, I know Python uses re, but regex as a name seems more explicit/discoverable/googleable.
• I'd like to add some example code under koto/test/libs, maybe you have some tests you built the lib against? No worries if not.
• Would it make sense to give Captures an iterable interface (like Matches) rather than making it a container? If map access is desirable then maybe .to_map() could be used on the iterator output?

I can take care of moving this forward so don't worry about making changes (unless you feel like it), although it won't be until after the weekend.

[jasal82]

Thanks for your comments. Here are my thoughts:

• Rename to regex sounds good, I tend to keep namespaces short, especially for things that are used very often. But I agree that it might be clearer. Then I'd also suggest to rename the regex() method to pattern() or new(). Not sure what the Koto convention is here.
• I have some basic tests that I can add.
• Users usually expect random access to the capture groups and map access on the named capture groups. Not every capture group is named, so I think the interface you suggest would feel rather awkward. There's also a capture_all method in the original regex crate (which is currently missing from the Koto module) which returns an iterator over the captures for all individual matches. There I would keep the iterator in Koto and have it dereference to a Captures.

[irh]

Hi @jasal82, just pinging to say I haven't forgotten about this PR, and I'll get around to it as soon as I can.

[irh]

@jasal82 I've updated the regex lib now to the point where I'd be happy to land it, if you have a moment could you take a look at the API changes I made? No problem if you don't have time, we can always revisit it later if you have notes.