Add variance back to Liskov combinators.

This partly reverts 227a19290b4c16bb9f7db4426045be2ad9a2ecf0.  The
soundness problem is explained in
[SI-7872](https://issues.scala-lang.org/browse/SI-7872).
Deprecate Liskov#inv.
Merge pull request #546 from S11001001/topic/si-7872

Don't break SI-7872