[pcc] use the direct CallContext methods GH #1083

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

in fill_params() it is safe to use CallContext directly, as
it assumes either NULL or CallContext, but never PMCNULL.

all tests still passing, 1.63% faster.

Signed-off-by: Reini Urban <rurban@cpanel.net>