Add a C style vtable to Managed and use it for markObjects

Add a C style vtable to Managed and use it for markObjects

We need to replace the C++ vtable with a home made one for additional
flexibility. This will help us deal properly with primitive this
values, and allow us to enable certain optimisations where we can
change behavior at runtime (e.g. use optimised lookups as long as
we don't have any accessor properties defined).

Change-Id: I6a3852692bdc5c4f0bde05f6ff2b296013ba47e5
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>