dynstack: pushing a prompt no longer allocates memory

dynstack: pushing a prompt no longer allocates memory

* libguile/control.h: Remove scm_t_prompt_registers and
  scm_c_make_prompt_registers.
  (scm_c_abort): Take a pointer to a jmpbuf instead of a cookie.  It
  will serve the same purpose.
* libguile/control.c (reify_partial_continuation, scm_at_abort): Adapt
  to new prompt representation.

* libguile/dynstack.h:
* libguile/dynstack.c (scm_dynstack_push_prompt): Prompts now have 5
  words instead of 2, as they now push the fp, sp, ip, and jmpbuf on the
  stack separately.  This avoids allocation.
  (scm_dynstack_find_prompt): Likewise, add return values for fp, sp,
  etc.
  (scm_dynstack_wind_prompt): Replaces scm_dynstack_relocate_prompt.

* libguile/eval.c (eval):
* libguile/stacks.c (find_prompt):
* libguile/throw.c (pre_init_catch): Adapt to the new prompt mechanism.

* libguile/vm-engine.c (vm_engine): Setjmp an on-stack jmpbuf every time
  the VM enters.  We can then re-use that jmpbuf for all prompts in that
  invocation.

* libguile/vm-i-system.c (partial_cont_call): Adapt to change in prompt
  representation.  We don't need to wind here any more, since we pass in
  the prompt's jmpbuf.
  (prompt): Adapt to scm_dynstack_push_prompt change.
  (abort): Adapt to vm_abort change.

* libguile/vm.h (struct scm_vm): No more cookie.

* libguile/vm.c (vm_abort): Adapt to scm_c_abort change.
  (vm_reinstate_partial_continuation): Rewind the dynamic stack here,
  now that we do have a valid jmpbuf.
  (make_vm): No need to initialize a cookie.