The instance for Delta[Set[T], ...] should always depend on the instance on T

Doing otherwise is only an optimization, which is not appropriate at this point
of the design process. Hence remove the other implementation.
DeltaValue: remove NoChange + misc

Misc cleanups/changes (I don't remember the reason for them).
Change interface implemented by DeltaValue
Cleanup comments
Update IDEA workspace
Revert "Setup to measure again index construction time"

This reverts commit 9c37b90003572002ed4e1e32dc8b029e6ba9cfc9 - it was intended
to be very temporary, to allow measuring only the index construction time (since
I already had the numbers for the other measurements).