stage2: update for new usingnamespace semantics

src/value.zig

@@ -315,7 +315,7 @@ pub const Value = extern union {
     /// Prefer `castTag` to this.
     pub fn cast(self: Value, comptime T: type) ?*T {
         if (@hasField(T, "base_tag")) {
-            return base.castTag(T.base_tag);
+            return self.castTag(T.base_tag);
         }
         if (self.tag_if_small_enough < Tag.no_payload_count) {
             return null;