Initial reorg.

This is largely the same as #305, but updated for the current tree.

crates/wasi-common/src/fs/readdir.rs

@@ -0,0 +1,32 @@
+use crate::fs::DirEntry;
+use crate::{hostcalls, wasi};
+
+/// Iterator over the entries in a directory.
+///
+/// This corresponds to [`std::fs::ReadDir`].
+///
+/// TODO: Not yet implemented.
+///
+/// [`std::fs::ReadDir`]: https://doc.rust-lang.org/std/fs/struct.ReadDir.html
+pub struct ReadDir {
+    fd: wasi::__wasi_fd_t,
+}
+
+impl ReadDir {
+    /// Constructs a new instance of `Self` from the given raw WASI file descriptor.
+    pub unsafe fn from_raw_wasi_fd(fd: wasi::__wasi_fd_t) -> Self {
+        Self { fd }
+    }
+}
+
+/// TODO: Not yet implemented.
+impl Iterator for ReadDir {
+    type Item = DirEntry;
+
+    /// TODO: Not yet implemented.
+    fn next(&mut self) -> Option<Self::Item> {
+        unimplemented!("ReadDir::next");
+    }
+}
+
+// TODO: impl Debug for ReadDir