Fix clippy::needless_lifetimes warning (#147)

```
error: the following explicit lifetimes could be elided: 'a
   --> src/lib.rs:212:6
    |
212 | impl<'a, T> Clone for Iter<'a, T> {
    |      ^^                    ^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
    = note: `-D clippy::needless-lifetimes` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::needless_lifetimes)]`
help: elide the lifetimes
    |
212 - impl<'a, T> Clone for Iter<'a, T> {
212 + impl<T> Clone for Iter<'_, T> {
    |
```

src/lib.rs

@@ -209,7 +209,7 @@ pub struct Iter<'a, T> {
     len: usize,
 }
 
-impl<'a, T> Clone for Iter<'a, T> {
+impl<T> Clone for Iter<'_, T> {
     fn clone(&self) -> Self {
         Self {
             entries: self.entries.clone(),