Replay: save `itm_decode` structs instead of `api` structs

[tmplt]

At this point, itm_decode is much more stable than the RTIC Scope API. To allow older traces to be replayed with future releases of cargo-rtic-scope, itm_decode structures should be saved to file for replay instead of API structures. Replay files will then only bit-rot when itm_decode breaks the API.

[tmplt]

Even better would perhaps be to save the raw trace stream to file such that future releases of itm-decode (bug fixes, new stuctures) can be applied to old traces. This will become a bit difficult now that itm-decode takes a std::io::Read as input instead of raw bytes. Perhaps can push arbitrary data via Decoder::get_mut? But that gives us a std::io::Read; I don't think we are allowed to write to it, unless we create some dumb type that implements Read on a Vec<u8> type.

[tmplt]

Let's wait for rust-embedded/itm#41 in either case.

[tmplt]

Turns out that we already serialize itm structures to file.