Compose

After discussion with @patrickt, I’ve opted to use the `>>>` and `<<<` operators as defined in Haskell’s [Control.Arrow](http://hackage.haskell.org/package/base-4.7.0.1/docs/Control-Arrow.html#g:2).

_Rationale:_ (f . g)(x) captures “f(g(x))” reasonably well, but this can make for poor readability. Since `.` doesn’t indicate directionality at all, it’s easy to forget that its operands (supplied left to right) are applied right to left. In contrast, `<<<` and `>>>` indicate the direction of application: `<<<` is right-to-left, and `>>>` is left-to-right.

Further, since Swift allows type-based name overloading, we can extend this to apply to monads and arrows should they ever be definable in Swift’s type system, with the same semantics.