move module-public-interface to C, and expose it as C API

* libguile/modules.h:
* libguile/modules.c:
* ice-9/boot-9.scm (module-public-interface): Move definition of
  module-public-interface to C, where it is now available as
  scm_module_public_interface ().

ice-9/boot-9.scm

@@ -1783,8 +1783,7 @@
 ;;; The directory of all modules and the standard root module.
 ;;;
 
-(define (module-public-interface m)
+;; module-public-interface is defined in C.
-  (module-ref m '%module-public-interface #f))
 (define (set-module-public-interface! m i)
   (module-define! m '%module-public-interface i))
 (define (set-system-module! m s)

libguile/modules.c

@@ -620,6 +620,25 @@ SCM_DEFINE (scm_module_import_interface, "module-import-interface", 2, 0, 0,
 }
 #undef FUNC_NAME
 
+SCM_SYMBOL (sym_sys_module_public_interface, "%module-public-interface");
+
+SCM_DEFINE (scm_module_public_interface, "module-public-interface", 1, 0, 0,
+	    (SCM module),
+	    "Return the public interface of @var{module}.\n\n"
+            "If @var{module} has no public interface, @code{#f} is returned.")
+#define FUNC_NAME s_scm_module_public_interface
+{
+  SCM var;
+
+  SCM_VALIDATE_MODULE (1, module);
+  var = scm_module_local_variable (module, sym_sys_module_public_interface);
+  if (scm_is_true (var))
+    return SCM_VARIABLE_REF (var);
+  else
+    return SCM_BOOL_F;
+}
+#undef FUNC_NAME
+
 /* scm_sym2var
  *
  * looks up the variable bound to SYM according to PROC.  PROC should be

libguile/modules.h

@@ -100,6 +100,7 @@ SCM_API void scm_c_export (const char *name, ...);
 
 SCM_API SCM scm_sym2var (SCM sym, SCM thunk, SCM definep);
 
+SCM_API SCM scm_module_public_interface (SCM module);
 SCM_API SCM scm_module_import_interface (SCM module, SCM sym);
 SCM_API SCM scm_module_lookup_closure (SCM module);
 SCM_API SCM scm_module_transformer (SCM module);