* modules.h, modules.c: Moved around a lot of code so that

* modules.h, modules.c: Moved around a lot of code so that
deprecated features appear at the bottom.
(root_module_lookup_closure, scm_sym_app, scm_sym_modules,
module_prefix, make_modules_in_var, beautify_user_module_x_var,
scm_the_root_module, scm_make_module, scm_ensure_user_module,
scm_load_scheme_module): Deprecated.
(scm_system_module_env_p): Return SCM_BOOL_T directly for
environments corresponding to the root module.
(convert_module_name, scm_c_resolve_module,
scm_c_call_with_current_module, scm_c_define_module,
scm_c_use_module, scm_c_export): New.
(the_root_module): New static variant of scm_the_root_module.  Use
it everywhere instead of scm_the_root_module.