made inline versions of some hot functions.  Changed api to use
internal Object pointers rather than void pointers.

Speeds up getElementById by ~7%.
Review URL: http://codereview.chromium.org/173348

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2761 ce2b1a6d-e550-0410-aec6-3dcde31c8c00