Correct function signature for Parrot_ext_call and refactor a bit
Correct function signature for Parrot_ext_call and refactor a bit

The sig for Parrot_ext_call should be Pf->P, which returns a PMC, not Pf->
which returns nothing. Also, refactor create_* functions so that the global
interpreter does not have to be passed in.

leto/plparrot,leto/plparrot,leto/plparrot