Validate UFOv3 field values where appropriate

[epilys]

Some UFOv3 fields can have only specific values (for example, lists of bit flags with specific values or lists with specific length).

These can be implemented as serde's deserialize_with field attributes. There are already similar examples in the ufo module [0]

[0]: example https://github.com/epilys/gerb/blob/5911e42d5fd0a13cb06a68d3fb67ff664305eb57/src/ufo/glif.rs#L145