Rework hook dispatch mechanism

Rework hook dispatch mechanism

* libguile/vm-engine.c (RUN_HOOK, RUN_HOOK0, RUN_HOOK1): Rework to
  dispatch through specific per-hook procedures.  Might reduce register
  pressure in the VM.

* libguile/vm.c (vm_dispatch_apply_hook):
  (vm_dispatch_push_continuation_hook):
  (vm_dispatch_pop_continuation_hook):
  (vm_dispatch_next_hook):
  (vm_dispatch_abort_hook):
  (vm_dispatch_restore_continuation_hook): New internal helpers.