rename "closure-ref" to "free-ref"; s/vars/variables/ in some names

rename "closure-ref" to "free-ref"; s/vars/variables/ in some names

* libguile/programs.h:
* libguile/programs.c: (SCM_PROGRAM_FREE_VARIABLES): Rename from
  SCM_PROGRAM_FREE_VARS. Callers changed.
* libguile/programs.c (scm_make_program): Rename arg to
  "free_variables".
  (scm_program_free_variables): Rename from program-free-vars.

* libguile/vm-engine.h:
* libguile/vm-engine.c (VM_CHECK_FREE_VARIABLES): Rename from
  VM_CHECK_CLOSURE.
  (vm_engine, CACHE_PROGRAM): Rename closure and closure_count to free_vars and
  free_vars_vount.

* libguile/vm-i-system.c (FREE_VARIABLE_REF): Rename from CLOSURE_REF.
  (free-ref, free-boxed-ref, free-boxed-set): Rename from closure-ref,
  closure-boxed-ref, closure-boxed-set.
  (make-closure): Renamed from make-closure2.

* module/language/glil/compile-assembly.scm (glil->assembly): Hack to
  never write out the the old "make-closure" instruction. Will fix
  better later. Change to emit free-ref etc instead of closure-ref.

* module/language/tree-il/compile-glil.scm (flatten): Emit make-closure
  instead of make-closure2, now that the old make-closure is gone.

* module/system/vm/program.scm (system): Rename program-free-vars to
  program-free-variables.

* test-suite/tests/tree-il.test ("lambda"): Update for make-closure.