`CircuitType` in verification key? Something else?

[codygunton]

The old ComposerType was very much overloaded and overused. With a much more complex stack of proving systems being introduced, we have begun to unwind some of its various responsibilities. One of those responsibilities is to lock a verification key to a particular proving system. We should figure out if/when/how to do this. This may not be the responsibility of Barretenberg.