Improve keyed loads on strings by using a new stub.

Improve keyed loads on strings by using a new stub.

Instead of going through a runtime function for keyed loads
on strings we invoke a separate specialized stub that
assumes string as receiver type and the key to be a number.

The stub calls a JS builtin function to return the corresponding one-character string.

Review URL: http://codereview.chromium.org/521041

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3556 ce2b1a6d-e550-0410-aec6-3dcde31c8c00