incr.comp.: Compute fingerprint for all query results.
This PR enables query result fingerprinting in incremental mode. This is an essential piece of infrastructure for red/green tracking. We don't do anything with the fingerprints yet but merging the infrastructure should protect it from bit-rotting and will make it easier to start measuring its performance impact (and thus let us determine if we should switch to a faster hashing algorithm rather sooner than later).
Note, this PR also includes the changes from https://github.com/rust-lang/rust/pull/43887 which I'm therefore closing. No need to re-review the first commit though.
r? @nikomatsakis
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