Thanks to Aleš Bizjak.

This allows us to factor out properties about connectives that
commute with the big operators.

Used in iRevert, iClear, iFrame, and for generalizing the IH in
iInduction and iLöb.

This way we can use uPred_valid for validity of uPreds, which
more sense.

As proposed by JH Jourdan in issue 34.

We only had the step-indexed version before. Unfortunately, the non
step-indexed version does not follow from the step-indexed version.

This also solves a name clash with the extension order of CMRAs.