variable payload length

[miDeb]

This PR first introduces a new trait ByteCodec (open to other naming suggestions).
It allows to serialize to a variable number of bytes, as well as deserialize from an arbitrary number of bytes. It also contains a MAX_SERIALIZED_SIZE to check size assumptions at compile time. zerocopy-able types automatically implement this trait.
Also, a new proc macro is introduced to automatically implement ByteCodec for enums or structs that consist of fields that implement ByteCodec themselves.

I added a new file can_data.rs with some special data types that we send over CAN. One example is CanDataValue. It has a custom implementation of ByteCodec, because it does not encode the discriminant. When deserializing, we then don't know what value was originally encoded, so I added a new variant Raw, that is created when deserializing. It can then be converted to a regular variant by supplying the actual data type (convert_from_raw). Other types with custom serialization/deserialization I added are CanString (null-terminated ascii string limited to a certain length), PackedCanDataValues (for TelemetryGroupUpdatePayload) and NonNullCanBytes (for the field_ids in TelemetryGroupDefinitionPayload).

I removed the CanMessageFrame type, and instead now serialize to socketcan::CanFdFrames from the socketcan crate. Though that might also live in FerroFlow instead, I don't know...

I also updated the spec to include info about variable length encodings.

Spec diff

[raffael0]

Looks really great! Thank you for the work.
This will really simplify the implementation.
There were some small things where I think we could improve the performance, but I don't want to do any more bike shedding and I think it'll be fine for now (At least until we run performance tests).

Feel free to merge!