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).

Also inclusdes changes prompted by reviews/pulls:

Convert spaces-->tabs in makefiles.
Move things (again) to std/regex/internal.
Use new package(std.regex) feature.
Remove C-style arrays (some other pull against regex.d).
Merge pull request #2412 from DmitryOlshansky/regex-pkg

Split up the intertwined mess of std.regex.