Inspired by Data.Eq from Haskell.

Instances must obey the following laws:

1. Reflexivity: S.equals(a, a) === true
2. Symmetry: S.equals(a, b) === S.equals(b, a)
3. Transitivity: if S.equals(a, b) and S.equals(b, c), then S.equals(a, c)

Equivalent with the Setoid type-class in the static-land spec.