[pmc2c] Faster CallContext ATTR accessors + non-static for GH #1083

Omit the obj check in the CallContext ATTR accessors.

Put CallContext non-static decl into .h as they are not static anymore,
so that we can call them now directly internally in src/call/*.c
to avoid VTABLE redirection for those.

work together with Reini Urban
[pcc] use the direct CallContext methods GH #1083

and do not use the CallContext vtable methods anymore.
also inline one Continuation_set_pointer call.

all tests still passing, 2.98% faster.