Review overriddable vtable interfaces

[whiteknight]

we should go through the list of vtable interfaces to make sure we are allowing overrides on all the interfaces that we need to use. Here is a list of vtable interfaces that currently don't appear to support overriding:

init
init_pmc
destroy*
mark*
get_attr_keyed
set_attr_keyed
can
isa_pmc
isa
does
does_pmc
type*
invoke (we have it, but it doesn't work correctly)
visit*
thaw*
thawfinish*
share_ro*

I've marked with a * some of the vtable entries above that really can't be overridden in PIR or that don't make sense to be. Are there any others from this list that should be overridable?

And while I'm at it, are there any methods currently that are overridable, but really shouldn't be?