Make the change type be generic over any T rather &T (#31)

This makes the interface of this crate more flexible as the utility
methods such as `iter_changes` now also work if a container does not
contain references.

examples/nonstring.rs

@@ -0,0 +1,13 @@
+use similar::{capture_diff_slices, Algorithm};
+
+fn main() {
+    let old = vec![1, 2, 3];
+    let new = vec![1, 2, 4];
+    let ops = capture_diff_slices(Algorithm::Myers, &old, &new);
+
+    for op in ops {
+        for change in op.iter_changes(&old, &new) {
+            println!("{:?}", change);
+        }
+    }
+}