Add an asyncFor macro or smthng.

[Q-Master]

There are many cases when it is good to asyncronously iterate through some iterator, especially in database drivers. It will be a good idea to make some sort of asyncFor macro to asyncmacros.nim to make this possible.
Something like this:

macro asyncFor*(iterClause: untyped, body: untyped): untyped =
  if iterClause.kind != nnkInfix or iterClause[0] != ident("in"):
    error("expected `x in y` after for")

  let cursor = iterClause[2]
  let itemName = iterClause[1]
  result = quote do:
    while true:
      let future = next(`cursor`)
      yield future
      if future.failed: 
        raise future.readError()
      let tu = future.read()
      if not tu[0]: break
      let `itemName` {.inject.} = tu[1]
      `body`

I've got that code somewhere from the forums and it seems working. The only thing is one should implement the proc next(cusor) with tuple[bool, T] as a result. There might be a better idea to implement, something like throwing an exception in the next() proc when iteration stopped.