Extract reusable functions in inheritance

[nojaf]

This uses the same

module Impl = (
  T: {
    type t
  },
) => {

trick like in #13.

So much code is deduplicated because of this.
@zth I strongly feel like we should do this.
Improving a binding in a base interface nicely ripples through to all the inheriting interfaces.

[nojaf]

So, I slept on this and maintenance-wise I feel like we should go for this.