This reduces the time take for mjsunit/limit-locals from 56.8s to 15.1s in debug
mode and from 12.0s to 1.6s in release mode.

Note that GrowableBitVector and BitVector should really be merged, and probably
have their allocation strategy parmeterized. The current state of affairs
involving tons of checks and delegation is extremely ugly, and it is far from
clear if all that special casing is a clear win. STL FTW! :-P

Review URL: https://codereview.chromium.org/11775016

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13327 ce2b1a6d-e550-0410-aec6-3dcde31c8c00