[pmc]: rpa: optimize slack filling with PMCNULL

Only fill the rest with PMCNULL when necessary, i.e. when
we set an index > size, the intermediate indices between size and index-1.
This saves a lot of slack filling code cycles.

Error on delete oob.
[pmc]: rpa: optimize delete_keyed_int

on index=0 just bump the offset as with shift,
else memmove en-block, not item by item in a loop.

This passes now most rpa related tests, just reverse has a conflict
with fpa. with reverse renamed to nreverse make test and benchmarking
is possible.