py, extmod: Introduce and use MP_FALLTHROUGH macro.

Newer GCC versions are able to warn about switch cases that fall
through.  This is usually a sign of a forgotten break statement, but in
the few cases where a fall through is intended we annotate it with this
macro to avoid the warning.

py/objset.c

@@ -445,6 +445,7 @@ STATIC mp_obj_t set_unary_op(mp_unary_op_t op, mp_obj_t self_in) {
                 }
                 return MP_OBJ_NEW_SMALL_INT(hash);
             }
+            MP_FALLTHROUGH
         #endif
         default:
             return MP_OBJ_NULL;      // op not supported