Consider renaming invariant to something like prepost

[ErichDonGubler]

This would be a breaking change, but a case was made by @eternaleye on Reddit that effectively argues that invariant is a misnomer if reduced to a pre- and post-condition check, instead of an exhaustive check that is rigorously maintained between all operations.

Basically, we don't want to convey an incorrect notion of what the invariant block actually does, so a different name would probably be best. Therefore, it's time to bikeshed on what the name should actually be for the transition away from invariant.

For completeness, tests and documentation will have to be updated in order to accomodate this change. It's also likely that an explanation of the divergence from the original inspiration in D will be necessary, as an educational item. :)