Note [Storing compatibility]

During axiom application, we need to be aware of which branches are compatible
with which others. The full explanation is in Note [Compatibility] in
GHc.Core.FamInstEnv. (The code is placed there to avoid a dependency from
GHC.Core.Coercion.Axiom on the unification algorithm.) Although we could
theoretically compute compatibility on the fly, this is silly, so we store it
in a CoAxiom.

Specifically, each branch refers to all other branches with which it is
incompatible. This list might well be empty, and it will always be for the
first branch of any axiom.

CoAxBranches that do not (yet) belong to a CoAxiom should have a panic thunk
stored in cab_incomps. The incompatibilities are properly a property of the
axiom as a whole, and they are computed only when the final axiom is built.

During serialization, the list is converted into a list of the indices
of the branches.