Fix clippy::assigning_clones warning

```
warning: assigning the result of `Clone::clone()` may be inefficient
    --> crossbeam-skiplist/src/base.rs:1958:17
     |
1958 |                 self.head = next_head.clone();
     |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `clone_from()`: `self.head.clone_from(&next_head)`
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#assigning_clones
     = note: `#[warn(clippy::assigning_clones)]` on by default

warning: assigning the result of `Clone::clone()` may be inefficient
    --> crossbeam-skiplist/src/base.rs:1985:17
     |
1985 |                 self.tail = next_tail.clone();
     |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `clone_from()`: `self.tail.clone_from(&next_tail)`
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#assigning_clones
```

crossbeam-skiplist/src/base.rs

@@ -1955,7 +1955,7 @@ where
                 None => self.range.end_bound(),
             };
             if below_upper_bound(&bound, h.key().borrow()) {
-                self.head = next_head.clone();
+                self.head.clone_from(&next_head);
                 next_head
             } else {
                 unsafe {
@@ -1982,7 +1982,7 @@ where
                 None => self.range.start_bound(),
             };
             if above_lower_bound(&bound, t.key().borrow()) {
-                self.tail = next_tail.clone();
+                self.tail.clone_from(&next_tail);
                 next_tail
             } else {
                 unsafe {