sample-app-jsaddle doesn't work under Firefox

[niteria]

Describe the bug
If you build the sample-app-jsaddle app and open it in Firefox the app will hang, loop sending many XHR requests and you get Error : Unexpected Duplicate. syncCallbacks=True nBatch=4 nExpected=6 on stdout.

This is not really an issue with miso, but with jsaddle. miso is triggering ghcjs/jsaddle#64

I'm opening this because I think this is easy to side-step in miso.

Additional context
The crux of the problem with jsaddle + Firefox is that Firefox (as opposed to Chrome) is strict about disallowing synchronous XHR requests.

Synchronous XHR requests are used in jsaddle to implement calls from the browser to the jsaddle backend for Haskell functions that return results.

Fortunately miso is written in a continuation-passing style, so as far as I can tell it has no need for such functions.

In fact, the only instance of using a sync function that I run into is in

miso/jsaddle-ffi/Miso/FFI.hs

Lines 221 to 227 in af33633

	delegateEvent :: JSVal -> JSVal -> JSM JSVal -> JSM ()
	delegateEvent mountPoint events getVTree = do
	cb' <- function $ \_ _ [continuation] -> do
	res <- getVTree
	_ <- call continuation global res
	pure ()
	delegateEvent' mountPoint events cb'

.

It's clearly not using the result, and changing from function to asyncFunction on this line

miso/jsaddle-ffi/Miso/FFI.hs

Line 223 in af33633

cb' <- function $ \_ _ [continuation] -> do

made the sample-app-jsaddle and my own app run successfully under jsaddle on Firefox.

[niteria]

Fixed by afe2536