Add a null decoder

[mlms13]

null is a valid JSON value, so it makes sense to have a decoder that succeeds only if it encounters the literal null value.

On its own, it probably isn't the most useful decoder, but it should allow us to rewrite the optional decoder as

let optional =
  decoder => alt(null |> map(() => None), decoder |> map(Option.pure))

[mlms13]

Not going to change the implementation of optional for 1.x because that would actually change the error that comes out of optional decoders. This will change in 2.0, though.