context: reintroduce `ctx.descendant` as deprecated Reintroduce `ctx.descendant` to help extensions authors update their extensions in order to use the new API.

obsolete: explode if metadata contains invalid UTF-8 sequence (API) The current metadata API can be a source of bugs since it forces callers to process encoding conversion by themselves. So let's make it reject bad data as a last ditch. I assume there's no metadata field which is supposed to store arbitrary BLOB like transplant_source.