Add Loop

Suggested:

``` purescript
class (Category a, Strong a) <= Loop a where
  loop :: forall b c d. a (Tuple b d) (Tuple c d) -> a b c
```

with laws (cribbed from https://hackage.haskell.org/package/base-4.7.0.2/docs/Control-Arrow.html#t:ArrowLoop):

``` purescript
loop (f <<< first h) = loop f <<< h
loop (f >>> first h) = loop f >>> h
```

.. and the rest which I haven't figured out yet (TODO).
