Implementation of "ure" module using re1.5 (review only)

This implements subset of "re" module functionality using tiny regexp engine https://github.com/pfalcon/re1.5 (more info in #13).

Besides just reviewing code, following should be decided before merging:

1. Which of multiple backends to use. I randomly chose "recursive" backend, it should be good enough for starters, and further work can be done later, but if there're immediate better ideas, they're welcome.
2. How to host re1.5 lib. The lib offers only the most basic regex and integration features, and would need to undergo many more changes going forward. Still, as someone who already proposed a lot of structural changes and separation, I'm not going to further this and propose to reference it is as a git submodule. Originally intended approach of importing the lib into the codebase is proposed, but we should be aware that it will require syncing with upstream many times (which makes changing tracking a bit awkward/complicated).

Well, the simple logic is that string processing advances past last separator match. Zero match means infinite loop, so simple approach is 1) stop there; 2) it doesn't really make sense to do splitting on empty separators, so user simply shouldn't do that. (Note that "b?" is not empty separator).

Anyway, I wanted to code search()/match() and be done with it for initial version, but had an extra half hour before plane landing, so proceed further with simple conversion of algo from micropython-lib/re-pcre. It's not complete and will take some time to make it so.