continuations return multiple values on the stack

continuations return multiple values on the stack

* libguile/vm.h (struct scm_vm_cont): Instead of saving the "IP", save
  "RA" and "MVRA". That is, save singly-valued and multiply-valued
  return addresses, so that we can return multiple values on the stack.
  (scm_i_vm_reinstate_continuation): Remove.
* libguile/vm.c (vm_capture_continuation): Rename from capture_vm_cont,
  and change the prototype so we can capture the RA and MVRA, and so
  that tail calls to call/cc can capture a continuation without the
  call/cc application frame.
  (vm_return_to_continuation): Rename from reinstate_vm_cont, and take
  arguments to return to the continuation. Handles returning to single
  or multiple-value RA.
  (scm_i_vm_capture_continuation): Change to invoke
  vm_capture_continuation. Kept around for the benefit of make-stack.

* libguile/vm-i-system.c (continuation-call): Handle reinstatement of
  the VM stack, with arguments.
  (call/cc, tail-call/cc): Adapt to new vm_capture_continuation
  prototype. tail-call/cc captures tail continuations.

* libguile/stacks.c (scm_make_stack): Update for scm_vm_cont structure
  change.

* libguile/continuations.h (struct scm_contregs): Remove throw_value
  member, which was used to return a value to a continuation.
  (scm_i_check_continuation): New internal function, checks that a
  continuation may be reinstated.
  (scm_i_reinstate_continuation): Replaces scm_i_continuation_call; just
  reinstates the C stack.
  (scm_i_contregs_vm, scm_i_contregs_vm_cont): New internal accessors.
* libguile/continuations.c (scm_i_make_continuation): Return
  SCM_UNDEFINED if we are returning again.
  (grow_stack, copy_stack_and_call, scm_dynthrow): Remove extra arg, as
  vm opcodes handle value returns.
  (copy_stack): No need to instate VM continuation.
  (scm_i_reinstate_continuation): Adapt.