nested-ref et al use module-ref-submodule; add -module nested variants

nested-ref et al use module-ref-submodule; add -module nested variants

* module/ice-9/boot-9.scm: Update comments above nested-ref to include
  ref-module and define-module!.
  (nested-ref, nested-set!, nested-define!, nested-remove!): Use
  module-ref-submodule to traverse the module hierarchy.
  (nested-ref-module, nested-define-module!): New functions, like
  nested-ref and nested-define!, but operate on namespaces instead of
  values.
  (local-ref-module, local-define-module): New analogs of local-ref and
  local-define, but for namespaces.