All instruction pointers are now scm_t_uint32*

All instruction pointers are now scm_t_uint32*

* libguile/frames.h (SCM_FRAME_RTL_RETURN_ADDRESS)
  (SCM_FRAME_SET_RTL_RETURN_ADDRESS): Remove.  The variants without _RTL
  now work fine.
  (struct scm_vm_frame): Change the return address to be a
  scm_t_uint32*.
  (struct scm_frame): Change IP to be scm_t_uint32*.

* libguile/frames.c (scm_c_make_frame)
* libguile/control.c (reify_partial_continuation, scm_c_abort)
* libguile/dynstack.c (PROMPT_IP)
  (scm_dynstack_push_prompt, scm_dynstack_find_prompt)
* libguile/dynstack.h:
* libguile/vm-engine.c (SYNC_IP, RETURN_ONE_VALUE)
  (call, return-values, prompt): Adapt.