Remove MVRA accessors in libguile

Remove MVRA accessors in libguile

* libguile/vm.c (scm_i_vm_capture_stack): Remove MVRA argument, in
  preparation for removing MVRA from frames.
  (scm_i_capture_current_stack): Adapt to scm_i_vm_capture_stack
  change.
  (vm_reinstate_partial_continuation): Remove references to MVRA.

* libguile/vm.h (struct scm_vm_cont): Remove mvra member.

* libguile/vm-engine.c (call): Set MVRA to 0.  Will remove later.
  (return-values): Return to RA.
  (call/cc): Remove MVRA from capture call.

* libguile/frames.c:
* libguile/frames.h (SCM_FRAME_MV_RETURN_ADDRESS)
  (SCM_FRAME_RTL_MV_RETURN_ADDRESS, scm_frame_mv_return_address): Remove
  accessors.

* libguile/control.c (reify_partial_continuation): Adapt to
  scm_i_vm_capture_stack change.