We can only call malloc/free once per group and we can avoid scanning
through a list of NULLs if we keep unprocessed groups in the beginning.

I also changed the internal representation of implicit references to
hold a handle to the parent (instead of a direct pointer). The
prologue callback must not trigger a GC, but it's better to be safe.

Review URL: http://codereview.chromium.org/6800003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7521 ce2b1a6d-e550-0410-aec6-3dcde31c8c00