A decoder that can read from the incremental compilation cache. It is similar to the one
we use for crate metadata decoding in that it can rebase spans and eventually
will also handle things that contain Ty instances.
An encoder that can write to the incremental compilation cache.
Provides an interface to incremental compilation data cached from the
previous compilation session. This data will eventually include the results
of a few selected queries (like typeck and mir_optimized) and
any side effects that have been emitted during a query.