Scheme frame objects hold relative stack offsets

Scheme frame objects hold relative stack offsets

* libguile/frames.h: Wrap the C interface to VM frames in
  BUILDING_LIBGUILE.  Change VM frames to record relative offsets into a
  stack held by some other object, so that if the stack moves they will
  remain valid.
* libguile/frames.c (scm_c_make_frame): Remove offset argument.
  (scm_i_frame_offset): Instead, compute the offset from the stack
  holder.
  (scm_i_frame_stack_base): New helper.
  (scm_frame_previous): Adapt.

* libguile/stacks.c (scm_make_stack)
* libguile/vm.c (vm_dispatch_hook):
* libguile/continuations.c (scm_i_continuation_to_frame): Adapt.