[svn ci] added support for decorators to pmc methods

[jerry gay]

as of r20545, you can now decorate pmc methods to give the compiler a
hand in warning you about bad code, just like we've been doing
throughout parrot core (a.k.a. seatbelts.) for a list of decorators,
see include/parrot/compiler.h. for more info, see
docs/dev/seatbelts.pod

for example, in r20546, Null PMC's get_pointer method now looks like this:

  PARROT_CAN_RETURN_NULL
  void* get_pointer() {
      return PMCNULL;
  }

decorate the remainder of the pmc methods today! the holidays are coming fast.
~jerry