Use virtually dispatched specialized scavengers instead of single generic ScavengeObjectSlow implementation.

Rollback of r5041 with assertion checking callback alignment removed. Map space is iterated in a special fashion during scavenges so special callback alignment is not required.

Review URL: http://codereview.chromium.org/2950003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5047 ce2b1a6d-e550-0410-aec6-3dcde31c8c00