overload __iter__ in loop.Wait

This will automatically exit child tasks in case of close() or throw() on the waiting task, but only if run through `await` or `yield from`

src/apps/homescreen/layout_homescreen.py

@@ -15,8 +15,4 @@ async def animate_logo():
 
 
 async def layout_homescreen():
-    wait = loop.Wait([swipe_to_rotate(), animate_logo()])
+    await loop.Wait([swipe_to_rotate(), animate_logo()])
-    try:
-        await wait
-    finally:
-        wait.exit()

src/trezor/loop.py

@@ -194,3 +194,10 @@ class Wait(Syscall):
             if self.exit_others:
                 self.exit()
             schedule_task(self.callback, result)
+
+    def __iter__(self):
+        try:
+            return (yield self)
+        except:
+            self.exit()
+            raise