Signatures for monadic traversal.
As with Mappable, we define the signature of traversable structures in an arity-generic way, then specialise it for arity-0 and arity-1 types.
module type Generic = sig ... endGeneric describes monadic traversal on either an arity-0 or arity-1 type.
module type S0 = sig ... endS0 is the signature of a monadic traversal over arity-0 types.
module type S1 = sig ... endS1 is the signature of a monadic traversal over arity-1 types.
module type Basic_container0 = sig ... endBasic_container0 is the signature that traversable containers of arity 0 must implement.
module type Basic_container1 = sig ... endBasic_container1 is the signature that traversable containers of arity 1 must implement.
module type Generic_on_monad = sig ... endGeneric_on_monad extends Generic to contain various derived operators; we use it to derive the signatures of the various On_monad modules.
module type On_monad1 = sig ... endOn_monad1 extends Generic_on_monad with functionality that only works on arity-1 containers.
module type Generic_container = sig ... endGeneric_container is a generic interface for traversable containers, used to build Container0 (arity-0) and Container1 (arity-1).
module type S0_container = sig ... endS0_container is a generic interface for arity-0 traversable containers.
module type S1_container = sig ... endS1_container is a generic interface for arity-1 traversable containers. It also includes the extensions from Mappable.