* libguile/continuations.h:
* libguile/continuations.c (scm_i_continuation_to_frame): Operate on
  low-level C structures instead of heap objects.

* libguile/frames.h:
* libguile/frames.c (frame_offset, frame_stack_base): Const args.
  (scm_c_frame_closure): New helper.
  (scm_frame_procedure): Use the new helper.

* libguile/stacks.c (stack_depth, narrow_stack, scm_make_stack): Rework
  to avoid allocating frames as we traverse the stack, and to avoid an
  n**2 case where there are outer cuts.