Split up the intertwined mess of std.regex.

The docs and API still stay in one file.

With time and refactoring more internals may be
exposed such as parser, each engine explicitly
and the sample generator (generator.d).

One problem is open - I still have no idea how to "forward" documentation to package module from implementation modules. For now I just aliased the stuff and documented aliases, a poor substitute.

To reviewers: it may be a good time to poke around and see if something doesn't make any sense.
Most of the code is there since 2011, there must be something to improve.