libregex: Implement a subset of the GNU extensions

The entire patch-set is not yet mature enough for commit, but this usable
subset is generally enough for googletest to be happy with and mostly map to
some existing concepts, so they're not as invasive.

The specific changes included here are:

• Branching in BREs with \|
• \w and \W for :alnum: and [^[:alnum:]] respectively
• \s and \S for :space: and [^[:space:]] respectively
• Additional quantifiers in BREs, \? and \+ (self-explanatory)

There's some #ifdef'd out work for allowing empty branches as a match-all.
This is a feature that's under assessment... future work will determine
how standard this behavior is and act accordingly.