[MockProver] Consider supporting the assumption that non-zero fixed columns correspond to active gates

[therealyingtong]

Fixed columns can act as non-binary selectors. Consider supporting the assumption that a gate is activated when a fixed column in the gate is set to non-zero.

(Original discussion in zcash/orchard#67 (comment))

[daira]

I meant that it should be possible to define such a column as a selector. Not all fixed columns will be selectors. (For example, the fixed columns used for tables and for constants will not be.)