underlying string representation of the two strings involved.
- Renamed ascii and two byte string classes to sequential ascii and
  sequential two byte, and renamed IsAscii and friends to
  IsAsciiRepresentation.  This is to make a clear distinction between
  strings with an ascii/two-byte representation, of which there is
  four, and flat sequential ascii/two-byte string.


git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@474 ce2b1a6d-e550-0410-aec6-3dcde31c8c00