Move Managed::type and some flags into the vtable

Move Managed::type and some flags into the vtable

Move the type flag into the vtable to free up these
bits in the Managed class, and not have to set them
at object construction time.

As we often need to know whether a Managed object is a
Object, FunctionObject or String, add some bitflags to test
for these to the vtable.

Change-Id: I7d08ca044544debb307b55f124f34cb086ad9e84
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>