Change SCM_PACK_RTL macros to SCM_PACK_OP / SCM_PACK_OP_ARG

Change SCM_PACK_RTL macros to SCM_PACK_OP / SCM_PACK_OP_ARG

* libguile/instructions.h: Guard in BUILDING_LIBGUILE.
  (SCM_PACK_OP_24):
  (SCM_PACK_OP_8_8_8):
  (SCM_PACK_OP_8_16):
  (SCM_PACK_OP_16_8):
  (SCM_PACK_OP_12_12): Rename from SCM_PACK_RTL_*, and splice in the
  opcode.
  (SCM_PACK_OP_ARG_8_24): New helper.

* libguile/vm.c:
* libguile/gsubr.c:
* libguile/foreign.c:
* libguile/control.c:
* libguile/continuations.c: Adapt.