remove "externals" from the vm

remove "externals" from the vm

* libguile/frames.c (scm_frame_external_link): Removed.
* libguile/frames.h: No need to have the "external link" in the stack
  frame -- update macros to take the new situation into account.

* libguile/objcodes.h (struct scm_objcode): Rename the nexts field to
  "unused". In the future we can use it for nlocs, I think.
  (SCM_OBJCODE_NEXTS): removed.

* libguile/programs.h:
* libguile/programs.c (scm_make_program): Expect the third argument to
  be a vector of free variables, not a list of free variables.
  SCM_BOOL_F indicates no free variables, not SCM_EOL.
  (program_mark): Adapt.
  (scm_program_arity): No more nexts.
  (scm_program_free_vars): Replaces scm_program_externals.

* libguile/vm-engine.c (VM_CHECK_EXTERNAL)
  (vm_engine): No need for the "external" var.
* libguile/vm-engine.h (CACHE_PROGRAM): Update for SCM_PROGRAM_FREE_VARS
  instead of SCM_PROGRAM_EXTERNALS.
  (NEW_FRAME): Update for new frame size, and no need to cons up
  externals. Yay :)

* libguile/vm-i-loader.c (load-program): Update for scm_make_program.

* libguile/vm-i-system.c (external-ref, external-set): No more.
  (make-closure): No more.
  (goto/args): No need to re-cons externals here. Update for new stack
  frame size.
  (mv-call, return, return/values): Update for new frame size. No need
  to reinstate externals on return.

* libguile/vm.c (really_make_boot_program, scm_load_compiled_with_vm):
  Update for scm_make_program.
* module/language/objcode/spec.scm (objcode-env-externals): Treat '() as
  #f, for the externals. Need to clean this up later...
* module/system/vm/program.scm (arity:nexts): Remove. Rename
  program-external to program-free-vars.