Implemented a first working version of FutureStreams.

tests/async/tfuturestream.nim

@@ -4,16 +4,17 @@ var fs = newFutureStream[string]()
 
 proc alpha() {.async.} =
   for i in 0 .. 5:
-    fs.put($i)
     await sleepAsync(1000)
+    fs.put($i)
 
-  fs.complete()
+  fs.complete("Done")
 
 proc beta() {.async.} =
-  while not fs.finished():
+  while not fs.finished:
     echo(await fs.takeAsync())
 
   echo("Finished")
 
 asyncCheck alpha()
-asyncCheck beta()
+waitFor beta()
+