Clippy warning fix for async fn/trait

src/limiters.rs

@@ -4,7 +4,7 @@
 ///
 /// [`SlidingWindowRateLimiter`]: ../sliding_window/struct.SlidingWindowRateLimiter.html
 pub trait RateLimiter {
-    async fn wait_until_ready(&mut self);
+    fn wait_until_ready(&mut self) -> impl std::future::Future<Output = ()> + Send;
 }
 
 /// A cost-based rate limiter, where each call can be of a variable cost.
@@ -15,5 +15,5 @@ pub trait RateLimiter {
 ///
 ///  [`TokenBucketRateLimiter`]: ../token_bucket/struct.TokenBucketRateLimiter.html
 pub trait VariableCostRateLimiter {
-    async fn wait_with_cost(&mut self, cost: usize);
+    fn wait_with_cost(&mut self, cost: usize) -> impl std::future::Future<Output = ()> + Send;
 }