Support (the most common subset of) regular expressions

Avian's support for regular expressions was limited to relatively basic patterns without any special constructs such as the dot, the star etc.

This topic branch implements a virtual machine dedicated to matching regular expression patterns and a compiler supporting the most common features of regular expressions.

Theoretically, the performance should be better than Oracle Java's (at least according to the documentation used to implement the PikeVM) although I did not bother testing that.

While the support for regular expression pattern operators is pretty broad, not all features have been implemented (most notably, Unicode character classes and named groups. For a complete list, please see the second-to-last commit.

The missing features are easy to suupport, though. Most likely, not even the implementation of the Pike VM needs to be touched (except for possessive quantifiers). I just ran a little out of steam, but am of course willing to implement features as the need arises.