save-analysis: use a decoupled representation for dumped data
Closes#33348
This will probably break any tool relying on the csv backend of save_analysis, for the following reasons:
1. Dumped spans don't contain extents anymore (`Dump` uses `SpanData` now instead of internal `Span`s). In case we still want to dump extents we could add them to `SpanData`.
1. `DefId`s are no longer dumped as a pair of `(ref_id, ref_crate)`. Instead, they are dumped as a single `Id`.
@nrc You said something about storing the id in a `u64`, but you didn't explain why. I kept using `u32` in this branch but I can change it if you prefer that.
r? @nrc
By the way, the fact that this breaks tools relying on CSV may be a good occasion to start dumping CSV in a different way (i.e. using the serializer like in the JSON backend).
