[pmc] rpa: optimize splice shrink fast on offset
[pmc] rpa: optimize splice shrink fast on offset

if there is enough room at the left to fill in the elements
from from, adjust offset and jump to the fill routine

this optimization is not present in qrpa, and executed e.g.
in data_json.

tkob/parrot,youprofit/parrot,youprofit/parrot,youprofit/parrot,youprofit/parrot,tkob/parrot,tkob/parrot,tkob/parrot,tkob/parrot,youprofit/parrot,youprofit/parrot,tkob/parrot,youprofit/parrot,parrot/parrot,parrot/parrot,parrot/parrot,tkob/parrot,youprofit/parrot,parrot/parrot,tkob/parrot,parrot/parrot