uavcan.si: make sample types include unit types to enhance composability

[pavel-kirienko]

Is there a reason that every si/sample type re-implements the corresponding si/unit type, instead of combining the timestamp with the corresponding si/unit/ type?

Originally posted by @JacobCrabill in https://github.com/UAVCAN/public_regulated_data_types/pull/81#discussion_r501822164