zkvm: implement and, or, verify instructions

[cathieyun]

• change Constraint::{And, Or} to be a tuple of two boxed constraints, instead of a vector (since they'll only ever be created from two constraints anyways, and we can do the flattening in the verify function
• implement and, or using the new Constraint type
• implement verify: enter 2nd phase of 2-phase challenge API, in order to get z challenge. Then recursively "flatten" the constraint, until the base case Expression type, and combine the Expressions according to the rules until you have one Expression that represents the whole constraint. Then push that constraint onto the constraint system.

Question: I implemented the From<R1CSError> for VMError trait, but in verify it does not automatically convert R1CSError to VMError, so I have to do it explicitly. I thought this would be implicit because of the From trait?
Update: What I'd want is TryFrom, which is nightly-only. So I'll just do explicit conversion for now!

[oleganza]

LGTM!