The stub for calling RegExp directly now also handles two byte strings.

Support for flat cons strings added for both ascii and two byte.

Some code code simplifications and added a few constants.
Review URL: http://codereview.chromium.org/545151

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3678 ce2b1a6d-e550-0410-aec6-3dcde31c8c00