whiteknight/vtable_overrides branch

[Andrew Whitworth]

In the whiteknight/vtable_overrides branch I have added the ability to
override the isa, isa_pmc, and can vtables. Previously these things
were not able to be overridden from PIR. I wouldn't necessarily call
these operations extremely common, but there are places where the
ability to override these behaviors can be important.

All tests pass for me on that branch, including new tests I've added
just for the purpose. However, this does add a certain amount of
overhead by needing to lookup potential vtable overrides first before
carrying on with normal behavior. I suspect this is not going to be a
huge expenditure, but it is something and we need to be aware of it.

I would like some help both testing and benchmarking this branch
against master. I don't know what kind of benchmark represents a
"normal" use-case for isa, isa_pmc, and can vtables. I assume NQP,
Rakudo, and other HLLs make heavier use of these things than most of
the Parrot test suite does. If the results don't look too bad I would
like to discuss merging this branch. If we do see some unacceptable
slowdowns, we can scrap this or try to improve upon it.

Thanks,

--Andrew Whitworth