Provisionally export new API

ext/digest/digest.def

@@ -0,0 +1,3 @@
+EXPORTS
+Init_digest
+rb_digest_wrap_metadata

ext/digest/digest.h

@@ -81,6 +81,7 @@ rb_digest_make_metadata(const rb_digest_metadata_t *meta)
     static wrapper_func_type wrapper;
     if (!wrapper) {
         wrapper = (wrapper_func_type)rb_ext_resolve_symbol("digest.so", "rb_digest_wrap_metadata");
+        if (!wrapper) rb_raise(rb_eLoadError, "rb_digest_wrap_metadata not found");
     }
     return wrapper(meta);
 #else