Don't use string slices when processing RexExp replace.

Don't use string slices when processing RexExp replace.

String slices from RegExp replace results is now encoded in either one or two smis. Substrings are not used any more.

If the existing one smi encoding cannot hold the start/length information two smis are used the first having the negative length and the second having the start.

This is in preparation for removing string slices.
Review URL: http://codereview.chromium.org/342015

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3153 ce2b1a6d-e550-0410-aec6-3dcde31c8c00