Rename VMOBJECT_PTR to oop_t, and fix uses of pVMObject

oop_t is much less intrusive, and it also is more decoupled from the AbstractVMObject hierarchy. pVMObject as well as VMOBJECT_PTR are confusing.

Fix uses of pVMObject that should be oop_t, because any object can be used at this point.

Signed-off-by: Stefan Marr <git@stefan-marr.de>
Integers do not have fields, can't store into them

Signed-off-by: Stefan Marr <git@stefan-marr.de>
Should not need to use void*

Signed-off-by: Stefan Marr <git@stefan-marr.de>