Fix ambiguous lifetimes. (#16)

* Fix ambiguous lifetimes.

These types of ambiguous lifetimes may shortly be forbidden by
rust-lang/rust#117967

Co-authored-by: Daniel Henry-Mantilla <daniel.henry.mantilla@gmail.com>

src/lending_iterator/constructors/windows_mut_.rs

@@ -50,18 +50,18 @@ for
         &'next mut [T; WINDOW_SIZE]
     ;
 
-    fn next (
-        self: &'_ mut WindowsMut<&'lt mut [T], WINDOW_SIZE>,
-    ) -> Option<&'_ mut [T;  WINDOW_SIZE]>
+    fn next<'next> (
+        self: &'next mut WindowsMut<&'lt mut [T], WINDOW_SIZE>,
+    ) -> Option<&'next mut [T;  WINDOW_SIZE]>
     {
         self.nth(0)
     }
 
     #[inline]
-    fn nth (
-        self: &'_ mut WindowsMut<&'lt mut [T], WINDOW_SIZE>,
+    fn nth<'nth> (
+        self: &'nth mut WindowsMut<&'lt mut [T], WINDOW_SIZE>,
         n: usize,
-    ) -> Option<&'_ mut [T;  WINDOW_SIZE]>
+    ) -> Option<&'nth mut [T;  WINDOW_SIZE]>
     {
         let new_start = self.start.checked_add(n)?;
         let slice =