Add multi argument callback helpers

[hamishmack]

We should add something like this to GHCJS.Foreign.Callback (based on function from jsaddle):

syncCallbackMulti :: OnBlocked -> (JSVal -> [JSVal] -> IO ())
                               -> IO (Callback (JSVal -> JSVal -> IO ()), Object)
syncCallbackMulti onBlocked f = do
    callback <- syncCallback2 onBlocked $ \this args -> do
        rargs <- Array.toListIO (coerce args)
        f this rargs
    (callback,) <$> makeMultiArgCallback callback

foreign import javascript unsafe "$r = function () { $1(this, arguments); }"
    makeMultiArgCallback :: Callback (JSVal -> JSVal -> IO ()) -> IO Object

Returns two values. The first is just for use with releaseCallback, the second is to pass on to JavaScript. We should change it to something more meaningful than Object.

[hamishmack]

Not sure if we need the this argument (I have never used it in the jsaddle version), so we could probably get by with:

type CallbackMulti = Callback ([JSVal] -> IO ())

syncCallbackMulti :: OnBlocked -> ([JSVal] -> IO ())
                               -> IO (Callback (JSVal -> IO ()), CallbackMulti)
syncCallbackMulti onBlocked f = do
    callback <- syncCallback1 onBlocked $ \args ->
        Array.toListIO (coerce args) >>= f
    (callback,) <$> makeMultiArgCallback callback

foreign import javascript unsafe "$r = function () { $1(arguments); }"
    makeMultiArgCallback :: Callback (JSVal -> IO ()) -> IO CallbackMulti

[crocket]

I suggest documenting syncCallbackMulti, asyncCallbackMulti, and syncCallbackMulti' on README.

[crocket]

Is a polyvariadic function going to help here? I don't know much about polyvariadic functions.

[crocket]

@luite This is useful because http://electron.atom.io/docs/api/app/#event-certificate-error requires a callback of 6 arguments. Currently, ghcjs-base cannot handle event-certificate-error.