deprecate lookup closures

deprecate lookup closures

* libguile/deprecated.h (SCM_TOP_LEVEL_LOOKUP_CLOSURE):
* libguile/deprecated.c (scm_lookup_closure_module):
  (scm_module_lookup_closure):
  (scm_current_module_lookup_closure): Deprecate this part of the eval
  closure interface.  It was unused internally, after the scm_sym2var
  refactor.

* libguile/eval.h:
* libguile/modules.c:
* libguile/modules.h: Remove deprecated code.

* libguile/goops.c (scm_ensure_accessor): Use scm_module_variable
  instead of calling the lookup closure.  However I'm not sure that this
  code is used at all.