Clean up some messiness in Scopes.

Clean up some messiness in Scopes.

For some reason, the scope's arguments and arguments shadow were
variable proxies, which resulted in all references to the arguments
shadow being shared in the AST.  This makes it hard to put per-node
state on the AST nodes.

I took the opportunity to remove Variable::AsVariable which has
confused people in the past, and to rename Variable::slot to the more
accurate Variable::AsSlot.

Review URL: http://codereview.chromium.org/3432022

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5517 ce2b1a6d-e550-0410-aec6-3dcde31c8c00