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

gerb/src/ufo/glif.rs

Line 145 in 5911e42

#[serde(serialize_with = "smooth_serialize")]