module-{ref,define}-submodule use the submodules table

module-{ref,define}-submodule use the submodules table

* module/ice-9/boot-9.scm (module-ref-submodule)
  (module-define-submodule!): Implement in terms of the
  module-submodules table, instead of looking for bindings in the value
  namespace.

* module/ice-9/deprecated.scm (module-ref-submodule):
  (module-define-submodule!): Define deprecated versions of these that
  duplicate the submodules table in the normal values namespace, for
  back compatibility.