[pmc] Object.invoke as vtable override: fix unstable self heuristic

Change experimental usage of vtable overridden subs.
Always push self, it is always called in method semantics.
$P1($P1) is not needed and not stripped anymore.

Change t/pmc/parrotobject.t tests, and add t/pmc/object-meths.t tests.

Closes #1013. Thanks to Zefram
[test] Clarify Object.invoke access to self

Add to tests that we always have access to self within
Object.invoke'd methods, even if not explictly passed.
This was one unhandled concern in http://trac.parrot.org/parrot/ticket/103
and fixed with GH #1013.