Unify async macro and futures for js and native targets

tests/async/tjsandnativeasync.nim

@@ -0,0 +1,30 @@
+discard """
+  output: '''hi
+bye'''
+"""
+
+import async, times
+when defined(js):
+    proc sleepAsync(t: int): Future[void] =
+        var promise = newPromise() do(resolve: proc()):
+            {.emit: """
+            setTimeout(function(){
+                `resolve`();
+            }, `t`);
+            """.}
+        result = promise
+else:
+    from asyncdispatch import sleepAsync, waitFor
+
+proc foo() {.async.} =
+    echo "hi"
+    var s = epochTime()
+    await sleepAsync(500)
+    var e = epochTime()
+    doAssert(e - s > 0.1)
+    echo "bye"
+
+when defined(js):
+    discard foo()
+else:
+    waitFor foo()