Implement sync and datasync on files and directories.

On Windows, there doesn't appear to be a way to sync a directory, to
ensure that the directory entry for a file is sync'd. So for now, just
silently succeed. I've opened WebAssembly/wasi-filesystem#79 to track
this at the spec level.

src/lib.rs

@@ -847,14 +847,14 @@ pub unsafe extern "C" fn fd_readdir(
  let dir = state.get_dir(fd)?;
  iter = DirEntryIterator {
  state,
- cookie: 0,
+ cookie: wasi::DIRCOOKIE_START,
  use_cache: false,
  stream: DirEntryStream(wasi_filesystem::readdir(dir.fd)?),
  };
 
  // Skip to the entry that is requested by the `cookie`
  // parameter.
- for _ in 0..cookie {
+ for _ in wasi::DIRCOOKIE_START..cookie {
  match iter.next() {
  Some(Ok(_)) => {}
  Some(Err(e)) => return Err(e),
@@ -2288,7 +2288,7 @@ impl State {
  dirent_cache: DirentCache {
  stream: Cell::new(None),
  for_fd: Cell::new(0),
- cookie: Cell::new(0),
+ cookie: Cell::new(wasi::DIRCOOKIE_START),
  cached_dirent: Cell::new(wasi::Dirent {
  d_next: 0,
  d_ino: 0,