This commit removes from the `core` module the `Split` typeclass. This
includes the following changes:

* The `Arrow` typeclass is no longer a subtype of `Split`.
* The `split` operation, previously in the `Split` typeclass, is now
  integrated in the `Arrow` typeclass, with a default implementation.
* Following notation from the  `Control.Arrow` library for Haskell,
  we use the operator `***` as an alias for `split`.
* `SplitSyntax` is replaced with `ArrowSyntax`.
* We remove the `SplitLaws` and the `SplitTests`. In consequence,
  ArrowLaws does not inherit from SplitLaws anymore.
* We modify the instances for `Kleisli`. We remove the trait `KleisliSpli`,
  and we replace the _factory_ method that was generating it to generate
  a `KleisliCompose` instead.
* We remove the tests on the `SplitLaws` for Kleisli and CoKleisli

We add a type-class of commutative arrows, which are those in which
the `split` operation is commutative (can pair in any order).

We introduce a Commutative Monad Type-class, a subclass of Monad in
which the flatMap of independent operations is commutative.

We introduce some instances of CommutativeArrow for Kleisli, which
are based on CommutativeMonad.

We introduce a new type class, CommutativeFlatMap, to load the
commutativity law one level up.

We introduce the duals of Commutative Comonads and CoflatMap,
as the duals of commutative Flatmaps and Monads.
This is done to generate and test CommutativeArrow instances for the
Cokleisli datatype.

We complete the tests for the CommutativeArrow instance of Cokleisli.
To use it, we mark the Monad instance of `NonEmptyList` as a
Commutative Monad.