Implement Ord for Path

The ordering of path (as obtained when iterating over a directory) in Littlefs
is not exactly what is expected.

This implementation matches the ordering that  will be observed when iterating,
as described in littlefs-project/littlefs#923

src/path.rs

@@ -1,6 +1,9 @@
 //! Paths
 
-use core::{convert::TryFrom, fmt, iter::FusedIterator, marker::PhantomData, ops, ptr, slice, str};
+use core::{
+    cmp::Ordering, convert::TryFrom, fmt, iter::FusedIterator, marker::PhantomData, ops, ptr,
+    slice, str,
+};
 
 use cstr_core::CStr;
 use cty::{c_char, size_t};
@@ -20,6 +23,40 @@ pub struct Path {
     inner: CStr,
 }
 
+impl Path {
+    fn cmp(&self, other: &Path) -> Ordering {
+        let this = self.inner.to_bytes();
+        let other = other.inner.to_bytes();
+
+        let min_len = this.len().min(other.len());
+
+        match this[0..min_len].cmp(&other[0..min_len]) {
+            Ordering::Less => Ordering::Less,
+            Ordering::Greater => Ordering::Greater,
+            Ordering::Equal if this.len() != other.len() => {
+                if this.len() < other.len() {
+                    Ordering::Greater
+                } else {
+                    Ordering::Less
+                }
+            }
+            Ordering::Equal => Ordering::Equal,
+        }
+    }
+}
+
+impl Ord for Path {
+    fn cmp(&self, other: &Path) -> Ordering {
+        self.cmp(other)
+    }
+}
+
+impl PartialOrd<Path> for Path {
+    fn partial_cmp(&self, other: &Path) -> Option<Ordering> {
+        Some(Ord::cmp(self, other))
+    }
+}
+
 /// Iterator over the ancestors of a Path
 ///
 /// See documentation for [`Path::ancestors`][]
@@ -258,7 +295,7 @@ impl Path {
         let rk_path_bytes = self.as_ref()[..].as_bytes();
         match rk_path_bytes.iter().rposition(|x| *x == b'/') {
             Some(0) if rk_path_bytes.len() != 1 => {
-                return Some(PathBuf::from("/"));
+                Some(PathBuf::from("/"));
             }
             Some(slash_index) => {
                 // if we have a directory that ends with `/`,
@@ -750,4 +787,12 @@ mod tests {
         let path = path!("/some/path/.././file.extension/");
         assert_eq!(path.file_name(), None);
     }
+
+    #[test]
+    fn path_ord() {
+        assert!(path!("some_path_a") < path!("some_path_b"));
+        assert!(path!("some_path_b") > path!("some_path_a"));
+        assert!(path!("some_path") > path!("some_path_a"));
+        assert!(path!("some_path") > path!("some_path_b"));
+    }
 }