Note [Detailed InertCans Invariants]

The InertCans represents a collection of constraints with the following properties:

  * All canonical

  * No two dictionaries with the same head
  * No two CIrreds with the same type

  * Family equations inert wrt top-level family axioms

  * Dictionaries have no matching top-level instance

  * Given family or dictionary constraints don't mention touchable
    unification variables

  * Non-CEqCan constraints are fully rewritten with respect
    to the CEqCan equalities (modulo eqCanRewrite of course;
    eg a wanted cannot rewrite a given)

  * CEqCan equalities: see Note [inert_eqs: the inert equalities]
    Also see documentation in Constraint.Ct for a list of invariants