Add is_mutable() functions

Correspond to the make_mutable() ones.
diff --git a/src/hb-unicode.h b/src/hb-unicode.h
index a84a948..4edab48 100644
--- a/src/hb-unicode.h
+++ b/src/hb-unicode.h
@@ -201,6 +201,8 @@
 void
 hb_unicode_funcs_make_immutable (hb_unicode_funcs_t *ufuncs);
 
+hb_bool_t
+hb_unicode_funcs_is_immutable (hb_unicode_funcs_t *ufuncs);
 
 /*
  * funcs