Some paths are too long

For example `rustbus::wire::unmarshal::traits::Variant`. `Variant` is not even a trait, so this could be `rustbus::wire::unmarshal::Variant`. Same for `rustbus::wire::marshal::traits::Variant`.

Similarly `rustbus::wire::marshal::traits::SignatureBuffer` could be `rustbus::signature::Buffer`.

Start by locating the module definitions and references for rustbus::wire::unmarshal::traits::Variant, rustbus::wire::marshal::traits::Variant, and rustbus::wire::marshal::traits::SignatureBuffer. Trace how these public paths are exposed and used before evaluating the proposed shorter paths. Done means the relevant names are organized under the requested modules and the project still builds and tests successfully.

Written by the indexing model from the issue text.