Read write helpers (#896)

Provides async versions of read / write helpers being stabilized in `std`.

src/fs.rs

@@ -10,3 +10,4 @@
 pub use tokio_fs::{create_dir, create_dir_all, file, hard_link, metadata, os, read_dir, read_link};
 pub use tokio_fs::{remove_dir, remove_file, rename, set_permissions, symlink_metadata, File};
 pub use tokio_fs::OpenOptions;
+pub use tokio_fs::{read, write, ReadFile, WriteFile};