Add %? operator

[lehins]

There are many places where we protect against zero denominator when constructing a rational that has a property x % 0 == 0. We should create a helper function %? that takes care of this use case for us and use it in all places where this use case occurs.

[Soupstraw]

I started on this, but I think it'd be better to add a new type NonZeroRational which can only be constructed from a non-zero rational. Then %? can have the type Rational -> NonZeroRational -> Rational.

The reason that I think this'll be better is that we often divide by totalStake so if we changed the type of totalStake to NonZeroRational, we'd only have to prove that it's non-zero once and then we can divide by that however many times we want.

[Soupstraw]

Ok, I also tried the other approach, but I don't think there's a nice way to do it, so I'll just use Maybe.