Fix clippy::bool_to_int_with_if warning

```
warning: boolean to int conversion using if
  --> src/single.rs:93:9
   |
93 | /         if self.state.load(Ordering::SeqCst) & PUSHED == 0 {
94 | |             0
95 | |         } else {
96 | |             1
97 | |         }
   | |_________^ help: replace with from: `usize::from(self.state.load(Ordering::SeqCst) & PUSHED != 0)`
   |
   = note: `(self.state.load(Ordering::SeqCst) & PUSHED != 0) as usize` or `(self.state.load(Ordering::SeqCst) & PUSHED != 0).into()` can also be valid options
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#bool_to_int_with_if
   = note: `#[warn(clippy::bool_to_int_with_if)]` on by default
```

src/single.rs

@@ -90,11 +90,7 @@ impl<T> Single<T> {
 
     /// Returns the number of items in the queue.
     pub fn len(&self) -> usize {
-        if self.state.load(Ordering::SeqCst) & PUSHED == 0 {
-            0
-        } else {
-            1
-        }
+        usize::from(self.state.load(Ordering::SeqCst) & PUSHED != 0)
     }
 
     /// Returns `true` if the queue is empty.