trim our set of vm hooks

trim our set of vm hooks

* libguile/vm.h (SCM_VM_PUSH_CONTINUATION_HOOK)
  (SCM_VM_POP_CONTINUATION_HOOK): New hooks, to replace
  enter/exit/return.
  (SCM_VM_BOOT_HOOK, SCM_VM_HALT_HOOK, SCM_VM_BREAK_HOOK): Remove these
  useless hooks.

* libguile/vm.c (scm_vm_push_continuation_hook)
  (scm_vm_pop_continuation_hook): New accessors.

* libguile/vm-i-system.c: Remove boot, halt, break, enter, exit, and
  return hooks. Also remove the break instruction. Instead now when we
  push a new continuation onto the stack we call PUSH_CONTINUATION_HOOK,
  and when we pop via a return we call POP_CONTINUATION_HOOK. APPLY_HOOK
  is now decoupled from continuation pushes and pops.

* libguile/vm-engine.h:
* libguile/vm-engine.c: Adapt for hooks.

* module/system/vm/trace.scm (vm-trace): Adapt for hooks. Also revive
  the #:instructions? #t mode.

* module/system/vm/vm.scm: Adapt exports for new set of hooks.