Instead of weak handles external strings use a separate table.  This
table uses 5 times less memory than weak handles.  Moreover, since we
don't have to follow the weak handle callback protocol we can collect
the strings faster and even on scavenge collections.

Review URL: http://codereview.chromium.org/467037

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3439 ce2b1a6d-e550-0410-aec6-3dcde31c8c00