Path: Add ordering comparison functions

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

This implementation contains 2 comparision functions, one matching what is expected,
and one matching the iteration order of littlefs directories,
as described in littlefs-project/littlefs#923

The fact that directories are ordered is documented:
https://github.com/littlefs-project/littlefs/blob/f53a0cc961a8acac85f868b431d2f3e58e447ba3/SPEC.md?plain=1#L304

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 +23,66 @@ pub struct Path {
     inner: CStr,
 }
 
+impl Path {
+    /// Compare the path using their string representation
+    /// This comarison function as would be expected for a `String` type.
+    ///
+    /// <div class="warning">
+    ///   This ordering does not match the ordering obsvered when iterating over a directory.
+    ///
+    ///   See <a href="#method.cmp_lfs">cmp_lfs</a> and <a href = "https://github.com/littlefs-project/littlefs/issues/923">littlefs#923</a>.
+    /// </div>
+    ///
+    /// ```
+    ///# use std::cmp::Ordering;
+    ///# use littlefs2::path;
+    /// assert_eq!(path!("some_path_a").cmp_str(path!("some_path_b")), Ordering::Less);
+    /// assert_eq!(path!("some_path_b").cmp_str(path!("some_path_a")), Ordering::Greater);
+    /// assert_eq!(path!("some_path").cmp_str(path!("some_path_a")), Ordering::Less);
+    /// assert_eq!(path!("some_path").cmp_str(path!("some_path_b")), Ordering::Less);
+    /// assert_eq!(path!("some_path").cmp_str(path!("some_path")), Ordering::Equal);
+    ///```
+    pub fn cmp_str(&self, other: &Path) -> Ordering {
+        self.inner.cmp(&other.inner)
+    }
+
+    /// Compare the path using their string representation
+    ///
+    /// This comparison function matches the iteration order of `littlefs` when iterating over directory.
+    /// For more information, see [littlefs#923](https://github.com/littlefs-project/littlefs/issues/923)
+    ///
+    /// ```
+    ///# use std::cmp::Ordering;
+    ///# use littlefs2::path;
+    /// assert_eq!(path!("some_path_a").cmp_lfs(path!("some_path_b")), Ordering::Less);
+    /// assert_eq!(path!("some_path_b").cmp_lfs(path!("some_path_a")), Ordering::Greater);
+    /// assert_eq!(path!("some_path").cmp_lfs(path!("some_path_a")), Ordering::Greater);
+    /// assert_eq!(path!("some_path").cmp_lfs(path!("some_path_b")), Ordering::Greater);
+    /// assert_eq!(path!("some_path_a").cmp_lfs(path!("some_path")), Ordering::Less);
+    /// assert_eq!(path!("some_path_b").cmp_lfs(path!("some_path")), Ordering::Less);
+    /// assert_eq!(path!("some_path").cmp_lfs(path!("some_path")), Ordering::Equal);
+    ///```
+    pub fn cmp_lfs(&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,
+        }
+    }
+}
+
 /// Iterator over the ancestors of a Path
 ///
 /// See documentation for [`Path::ancestors`][]
@@ -587,6 +650,8 @@ pub type Result<T> = core::result::Result<T, Error>;
 
 #[cfg(test)]
 mod tests {
+    use core::cmp::Ordering;
+
     use super::{Path, PathBuf};
     use crate::path;