Previously, `ChallengeScalar` could use the operator traits defined on
the `F: Field` type it wrapped, due to its `impl Deref<Target = F>`.
This was technically ambiguous, and Rust 1.49.0 makes that ambiguity an
error.
We could fix this by adding operator impls with `ChallengeScalar` on the
RHS, but that would conflict with zcash/halo2#111. Instead we manually
dereference every challenge scalar when used in an arithmetic operation.
Once we refactor the permutation argument implementation to be integrated
as Vec<permutation::Proof>, we can change this again to just map from the
Vec<permutation::Argument> inside ConstraintSystem.
