Message serialization

[davidlenfesty]

The preference is to handle message deserialization entirely in Rust with proc macros.

Nunavut will generate the required Rust types, with some extra annotations we need to properly describe UAVCAN types (see https://github.com/UAVCAN/nunavut/issues/209 for tracking what this will look like).

These will also need full testing, similar to what is done for C serialization in nunavut.