New API for std.regex replace & match

This is no small chunk but a necessary one. Most of changes are documentation, refactoring and a bunch on new tests.

Related discussion:
http://forum.dlang.org/thread/khmt7k$253k$1@digitalmars.com

Fixes issue 9579 along the way.
http://d.puremagic.com/issues/show_bug.cgi?id=9579