refactor: add with_capacity

src/backends/memory.rs

@@ -17,7 +17,7 @@ where
     val: T,
 }
 
-/// Memory backend.
+/// Memory backend. O(n). This backend is not recommended for production use.
 ///
 /// New in version 0.8.0.
 #[derive(Clone, Default)]
@@ -40,6 +40,12 @@ where
         }
     }
 
+    pub fn with_capacity(capacity: usize) -> MemoryBackend<T, S> {
+        MemoryBackend {
+            queue: Arc::new(Mutex::new(RefCell::new(Vec::with_capacity(capacity)))),
+        }
+    }
+
     pub fn read(&self, score: &S) -> Vec<T> {
         self.queue
             .lock()