Do not create handles for values of internal fields---this operation is performance
critical and plain pointers are safe.

Appy the same approach to External wrapping and unwrapping.

Plus some minor refactorings.

Review URL: http://codereview.chromium.org/270085

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3064 ce2b1a6d-e550-0410-aec6-3dcde31c8c00