Advantage is that it's much easier to add new element types (like FAST_SMI_ELEMENTS), and that handling logic for each element kind is (more) consolidated.

Currently, only GetElementsWithReceiver uses the new encapsulation, but the goal is to move much more element functionality into the class incrementally.

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/7527001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8810 ce2b1a6d-e550-0410-aec6-3dcde31c8c00