Better escaped character support

You can now include \G at the start of the pattern,
or \X in the pattern,
or \K (anywhere, because it's basically just ignored!) in the pattern,
or \R in the pattern.

(Note: Some of this stuff isn't even mentioned on ruby-doc.org!!)

These missing features were picked up due to adding a more rigorous (complete) list of tests.

Also a couple of odd bug fixes.