Support async lambdas/closures
Many times I find myself writing loops around async calls. So I thought to simplify my code a bit and do:

```nim
import chronos

proc mapAsync*[T](self: openArray[T], op: proc (x: T): Future[void] {.closure, async.}): Future[void] {.async.} =
  var futures: seq[Future[void]]
  for i in self:
    let future = op(i)
    futures.add(future)
  await allFutures(futures)
```

Which gets me an error
```
.nimble/pkgs/chronos-3.0.11/chronos/asyncmacro2.nim(92, 12) Error: Cannot transform this node kind into an async proc. proc/method definition or lambda node expected.
```

Am I incorrectly defining my closure or is this feature not supported with chronos?