Add test for borrowed LinearMap key.

src/linear_map.rs

@@ -570,6 +570,18 @@ mod test {
         static mut _L: LinearMap<i32, i32, 8> = LinearMap::new();
     }
 
+    #[test]
+    fn borrow() {
+        use crate::String;
+
+        let mut map = LinearMap::<_, _, 8>::new();
+
+        let s = String::<64>::try_from("Hello, world!").unwrap();
+        map.insert(s, 42).unwrap();
+
+        assert_eq!(map.get("Hello, world!").unwrap(), &42);
+    }
+
     #[test]
     fn partial_eq() {
         {