Elm mailbox revamp

proposal.elm

-- a distilation of https://github.com/elm-lang/elm-plans/issues/7

{- Mailbox is renamed Dispatcher, although this name is the part I'm least certain about.
   The address field is replaced with a dispatch field.
   It's minor, but I've found it helpful to put dispatch second, since the signal is easier to explain.
-}
type alias Dispatcher a = 
    { signal : Signal a 
    , dispatch : a -> Message -- the big change
    }
{- same as the old mailbox function, just renamed -}
dispatcher : a -> Dispatcher a 

{- remains opaque as before -}
type Message = ...

{- Rather than taking an Address and a value, now takes a Message
   The x can be replaced by Never if we add that. -}
send : Message -> Task x ()

{- Dropped from the API: the Address type, the message function, and the forwardTo function.
  Html.Events will need to be changed to look more like Svg.Events,
  taking (a function that creates) a message, rather than an address and a value. -}
  

This seems excellent!

As someone who has only recently been studying Elm in a serious way, I find this really helpful, as forwardTo was quite confusing to me, and I have been doing functional(-like) programming with JavaScript and Clojure for more than a couple of years.