zserio [1] has the former at least. It isn't intended for the same use cases as protobuf/capnproto/flatbutter though; in particular it has no backward or forwards compatibility. But it's great for situations where you know exactly what software is used on both ends and you need small data and fast en-/decoding.
[1] http://zserio.org/doc/ZserioLanguageOverview.html#bit-field-...