PackfileDebug: Bring more sanity to VTABLEs
PackfileDebug: Bring more sanity to VTABLEs

- We have a custom mark VTABLE, so mention that in init.
- Shorten mark a little by getting all the attributes at once.
- The get_integer VTABLE is actually returning the number of
  elements in the PMC, so use the elements VTABLE.
- The line number is an integer so use get_integer_keyed_int
  This keeps around get_pmc_keyed_int as a wrapper.
- Refactor get_string_keyed_int in preparation for further
  changes.
parrot/parrot,tkob/parrot,FROGGS/parrot,FROGGS/parrot,FROGGS/parrot,youprofit/parrot,tkob/parrot,parrot/parrot,tkob/parrot,FROGGS/parrot,youprofit/parrot,FROGGS/parrot,tkob/parrot,youprofit/parrot,youprofit/parrot,parrot/parrot,tkob/parrot,youprofit/parrot,parrot/parrot,tkob/parrot,youprofit/parrot,FROGGS/parrot,youprofit/parrot,FROGGS/parrot,tkob/parrot,FROGGS/parrot,tkob/parrot,youprofit/parrot,parrot/parrot