Added Windows asyncfile implementation.

tests/async/tasyncfile.nim

@@ -0,0 +1,14 @@
+discard """
+  file: "tasyncexceptions.nim"
+  exitcode: 0
+"""
+import asyncfile, asyncdispatch, os
+
+proc main() {.async.} =
+  var file = openAsync(getTempDir() / "foobar.txt", fmReadWrite)
+  await file.write("test")
+  file.setFilePos(0)
+  let data = await file.readAll()
+  doAssert data == "test"
+
+waitFor main()