Use VTable#Init_Obj instead of setting refcount directly
Move VTable#Make_Obj and #Init_Obj to host language code

Now the core code doesn't access obj->ref directly.
Even more lazy allocation of Perl objects

Change handling of refcounts in Perl bindings. If the LSB of ref.count
is 1, no Perl object has been allocated yet, and the remaining bits are
used to store the refcount. If the LSB of ref.count is 0, a Perl object
has been allocated in ref.host_obj.

This makes it possible to only allocate a Perl object if we really need
to.