Heap frames have a "frame kind" bit

Heap frames have a "frame kind" bit

* libguile/frames.h (enum scm_vm_frame_kind, SCM_VM_FRAME_KIND)
  (scm_c_make_frame): Add a "frame kind" bit to the first word.  This
  will allow the "stack holder" to be a non-SCM object.

* libguile/continuations.c (scm_i_continuation_to_frame):
* libguile/frames.c (scm_c_make_frame, scm_frame_previous)
* libguile/stacks.c (scm_make_stack):
* libguile/vm.c (vm_dispatch_hook): Adapt frame creators to set the
  frame kind bit.