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).

You can try to make implications from https://docs.python.org/3/library/re.html#re.split : "Note that split will never split a string on an empty pattern match."