Improve performance of Array.prototype.join and String.prototype.substring

Improve performance of Array.prototype.join and String.prototype.substring
by tweaking the JavaScript implementation of these functions.
Review URL: http://codereview.chromium.org/519061

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3545 ce2b1a6d-e550-0410-aec6-3dcde31c8c00