Add <$> operator (map) to decode

[hamza0867]

This PR adds the <$> operator to decode in order to be able to use this haskell like format when decoding. For example:

type my_user =
  { name : string
  ; age : int
  }

let make_my_user  name age = { name, age }

let my_user_decoder : my_user decoder = 
  let open D in
  make_unregistered
  <$> field "name" string
  <*> field "age" int

[mattjbray]

Thanks!