fs: add remove_dir_all and RemoveDirAllFuture (#1325)

Adds the sister function to `remove_dir` and mirrors the `create_dir_all` that's already exposed.

tokio-fs/src/remove_dir_all.rs

@@ -0,0 +1,46 @@
+use std::fs;
+use std::future::Future;
+use std::io;
+use std::path::Path;
+use std::pin::Pin;
+use std::task::Context;
+use std::task::Poll;
+
+/// Removes a directory at this path, after removing all its contents. Use carefully!
+///
+/// This is an async version of [`std::fs::remove_dir_all`][std]
+///
+/// [std]: https://doc.rust-lang.org/std/fs/fn.remove_dir_all.html
+pub fn remove_dir_all<P: AsRef<Path>>(path: P) -> RemoveDirAllFuture<P> {
+    RemoveDirAllFuture::new(path)
+}
+
+/// Future returned by `remove_dir_all`.
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct RemoveDirAllFuture<P>
+where
+    P: AsRef<Path>,
+{
+    path: P,
+}
+
+impl<P> RemoveDirAllFuture<P>
+where
+    P: AsRef<Path>,
+{
+    fn new(path: P) -> RemoveDirAllFuture<P> {
+        RemoveDirAllFuture { path }
+    }
+}
+
+impl<P> Future for RemoveDirAllFuture<P>
+where
+    P: AsRef<Path>,
+{
+    type Output = io::Result<()>;
+
+    fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Self::Output> {
+        crate::blocking_io(|| fs::remove_dir_all(&self.path))
+    }
+}