Frame pointer points to local 0 instead of local 1

Frame pointer points to local 0 instead of local 1

* libguile/frames.h: Change so that fp points at local 0 instead of
  local 1, and clean up a bit.
  (struct scm_vm_frame): Remove program, and rename stack to locals.
  (SCM_FRAME_DATA_ADDRESS): Remove; it was redundant with
  SCM_FRAME_LOWER_ADDRESS.
  (SCM_FRAME_STACK_ADDRESS): Remove; replace with the new
  SCM_FRAME_LOCALS_ADDRESS.
  (SCM_FRAME_UPPER_ADDRESS): Remove; unused.
  (SCM_FRAME_NUM_LOCALS, SCM_FRAME_PREVIOUS_SP): New defines.
  (SCM_FRAME_BYTE_CAST, SCM_FRAME_STACK_CAST): Remove; unused;
  (SCM_FRAME_LOCAL): New define, replaces SCM_FRAME_VARIABLE.
  (SCM_FRAME_PROGRAM): Add cautionary commentary.

* libguile/frames.c: Adapt static asserts.
  (scm_frame_num_locals, scm_frame_local_ref, scm_frame_local_set_x):
  Adapt.  This means that frame-local-ref 0 now returns the procedure.

* libguile/vm-engine.c (ALLOC_FRAME, RESET_FRAME)
  (FRAME_LOCALS_COUNT, LOCAL_REF, LOCAL_SET, RETURN_VALUE_LIST): Adapt
  to change in fp.
  (LOCAL_ADDRESS): New helper.
  (POP_CONTINUATION_HOOK): Reimplement, taking the previous FP as an
  argument.
  (ABORT_CONTINUATION_HOOK): Reimplement, taking no arguments.
  (RETURN_ONE_VALUE): Reimplement.
  (RETURN_VALUE_LIST): Adapt to FP change.
  (halt, return-values, subr-call, foreign-call, prompt)
  (continuation-call, compose-continuation, call/cc, abort): Adapt to FP
  change, mostly via using LOCAL_ADDRESS, etc abstractions instead of
  using the raw frame pointer.

* libguile/control.c (reify_partial_continuation): Update for fp
  change.

* libguile/vm.c (vm_reinstate_partial_continuation): Adapt to removal of
  SCM_FRAME_UPPER_ADDRESS.

* module/system/vm/frame.scm (frame-call-representation): Adapt to
  frame-local-ref change.

* module/system/vm/trace.scm (print-return): Remove unused
  frame-num-locals call.