Add explicit nopcodes

Add explicit nopcodes

* libguile/vm-engine.c (VM_NAME): Add explicit nopcodes, later to be
  interspersed with others.  This will allow us some extensibility
  without always shuffling around opcodes.  Also avoid lazy
  initialization; have the linker do it for us.

* libguile/instructions.c (parse_instruction):
  (scm_instruction_list): Rework instruction parsing to avoid using
  malloc.  It would seem that this would fix some GC issue -- but who
  knows!