Merge branch 'jh/fractional' into 'master'

Fractional typeclass.

A typeclass for fractional assertions, that is assertions that depend on a fraction and that can be split.

This is used to derive generically a few other instances for framing , destructing, combining and spliting assertions of sums of fractions. I found it usefull when doing fraction-heavy proofs in LambdaRust.

The Right Way  To Do It would be to use a typeclass over the *predicate* itself. Unfortunately, the unification algorithm of typeclasses is not powerful enough to do the right beta-expansion that would expose the predicate applied to some fraction. Instead, the `Fractional` type class has as parameters both the predicate and the applied form that can be directly unified with the fractured assertion. Not very pretty.

I wonder whether I should split this into two type classes: the first one would depend only on the predicate and would actually state the fractionality of it, and the second would do the beta-expansion job. What do you think?

See merge request !23