Lenses for Dependent Maps

Implementation of some useful typeclasses and functions for using lenses to work with DMap structures.

So you can do awesome and delicious things like:

|
import Data.Dependent.Map.Lens (dat)

data MyDMap a where
  AString :: MyDMap String
  AInt    :: MyDMap Int

-- This is okay and sets the value at `AString` to be "fred"
ok = dat AString ?~ "Fred"

-- This is a type error because of the constraints of our DMap that carry through to our lenses! Yay!
no = dat AInt .~ "33"

-- Type safe as `DMap` guarantees that our value at position of `AInt` is an Int and our lenses maintain that invariant.
f = dix AInt %~ (+ 33)

-- You could also code golf this to, if you're into such things.
f = dix AInt +~ 33

The functions and typeclasses are all a shameless imitation of their counterparts in Control.Lens.At, with a D prefix. So ix becomes dix and the constraint At becomes DAt.

This is still very very very new package so it may be deprecated by a superior solution or change rapidly. :)