Encoded as normal ELF attributes, with proprietary tags.

I have an implementation of the full encode/decode in a private repo,
which I can share for porting into `object` if needed, but it seems a
bit out of scope.

E.g. a 6-bit PC-relative relocation into a 48-bit instruction has
`size == 6`, not `size == 48`.

As suggested by @philipc. Combined with the previous commit's change to
the meaning of `size` for these relocations, there is no longer any
ambiguity when converting between the 3-tuple and Rel::r_type constants.

As discussed in PR gimli-rs#593

The `_` pattern will take these matches now, so the result is still an
Error, just with a (probably) more truthful message.