Add `Birecursive` type class

[sellout]

The direct approach of

sealed trait Birecursive[T] extends Recursive[T] with Corecursive[T]

doesn’t seem to cut it when an abstract type member is in play, so we need something more clever.

I, @paulp, and @edmundnoble have taken swipes at this so far.

[sellout]

There is a gist with a minimal version of the problem.