make applicable smob calls cheaper, and fix a memory leak

make applicable smob calls cheaper, and fix a memory leak

* libguile/vm.c (prepare_smob_call): New helper.  Now, instead of making
  a per-smob trampoline, we will shuffle the smob into the args and use
  a gsubr.  This prevents a memory leak in which the trampolines, which
  were values in a weak-key table, were preventing the smobs from being
  collected.

* libguile/vm-i-system.c (call, tail-call, mv-call): Adapt to new smob
  application mechanism.
  (smob-call): Remove this instruction.

* libguile/smob.h (scm_smob_descriptor): Rename apply_trampoline_objcode
  to apply_trampoline.

* libguile/smob.c: Remove our own objcode trampolines in favor of using
  scm_c_make_gsubr.
  (scm_smob_prehistory): No more trampoline weak map.

* libguile/procprop.c (scm_i_procedure_arity): Adapt to applicable smob
  representation change.