call toplevel variables "toplevel", not "module"

call toplevel variables "toplevel", not "module"

* module/system/il/ghil.scm (ghil-var-for-set!, ghil-var-for-ref!)
  (ghil-var-define!): ghil-var-kind of a toplevel var is now 'toplevel.

* module/system/il/glil.scm: Renamve <glil-module> to <glil-toplevel>.
  Remove the unused `module' field. Remove the unused <glil-late-bound>
  type.

* module/system/il/compile.scm (make-glil-var): s/toplevel/module/

* module/system/vm/assemble.scm (<vlink-later>, <vdefine>): Remove the
  unused `module' parameters from these two types.
  (codegen, dump-object!): Adapt to module/toplevel name changes.