Modified RegexParsers to avoid forcing length() on the underlying CharSequence if possible.

Modified RegexParsers to avoid forcing length() on the underlying CharSequence if possible.

All existing tests pass. My first attempt broke /files/run/checked.scala and files/run/elidable.scala so I am fairly confident that this patch doesn't alter what the parser parses. However, I've not checked that it enables RegexParsers to run efficiently on huge PagedSeq objects.