deprecate scm_sym2var

deprecate scm_sym2var

* libguile/deprecated.h:
* libguile/deprecated.c (scm_sym2var): Deprecate this function.

* libguile/modules.h:
* libguile/modules.c (scm_module_ensure_local_variable): New public
  function, replacing scm_sym2var with a true definep, without going
  through eval closures (which are deprecated).
  (scm_current_module): Rework to do something sensible before modules
  are booted.
  (scm_module_lookup, scm_lookup): Refactor to use scm_module_variable.
  (scm_module_define, scm_define): Refactor to use
  scm_module_ensure_local_variable.

* libguile/vm-i-system.c (define!): Use scm_define.

* libguile/vm.c (resolve_variable): Use scm_module_lookup.

* libguile/macros.c (scm_make_syntax_transformer): Use
  scm_module_variable.

* libguile/gdbint.c (gdb_binding): Use scm_define.

* doc/ref/api-modules.texi (Accessing Modules from C): Add docs for
  scm_module_ensure_local_variable.