Upgrade Cadence to 0.0.43.

Closes #63.
Upgrade Proof to 0.0.50, Strata to 0.0.38.

Proof has a new scaffold. The Skip tests where all rewritten to
accommodate the new scaffold. Strata was upgraded at the same time
because the Skip tests use a scallfold wrapper from Strata that depends
on the new scaffold.

Closes #65.
Closes #64.