Change the BitVector utility class to allow allocation via 'new' in the
Zone.  Change the backing store to be always zone-allocated.

Review URL: http://codereview.chromium.org/730001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4071 ce2b1a6d-e550-0410-aec6-3dcde31c8c00