Add more comprehensive linear relationship tests for schnorr proofs

[marsella]

There are many forms of linear relationships that we don't currently test. For example

• Equality of values within the same message
• Any other linear relation of values within the same message
• addition check: secret values a, b, c=a + b. In this case, we would add the commitment scalars

There are probably others.

[marsella]

Note that linear relations in a single message might require a new public method to get a random commitment scalar. I think they're assumed to be generated internally for now.