This PR makes `mvcgen` produce deterministic case labels for the
generated VCs. Invariants will be named `inv<n>` and every other VC will
be named `vc<n>.*`, where the `*` part serves as a loose indication of
provenance.
This PR improves the API for invariants and postconditions and as such
introduces a few breaking changes to the existing pre-release API around
`Std.Do`. It also adds Markus Himmel's `pairsSumToZero` example as a
test case.
