Please define Predicate to not allow patch version without minor version

[joshtriplett]

The definition of Predicate includes an Option<u64> for minor and for patch; that type would allow a predicate with a patch but no minor. That causes a match expression on a Predicate to complain about missing cases. Please consider using an enum that doesn't allow that case; for instance:

enum Something { M(u64), MM(u64,u64), MMP(u64,u64,u64) }

[dtolnay]

On balance, I think the current representation is better.