`postcondition` is `Property`

[jasagredo]

This PR removes PostconditionM therefore disallowing monadic access to the SUT in postcondition checking.

It also refines the haddocks in monitoring as it is now redundant once liftProperty (extracted from QSM) exists. Any monitoring that was happening before can now be supplied inside postcondition or postconditionOnFailure. monitoringFailure remains unaltered because in its branch no postcondition is evaluated.

Checklist:

• [ ] Check source-code formatting is consistent