feat: unify optimize and optimize_with via blanket Objective impl

- Add blanket `impl Objective<V> for Fn(&mut Trial) -> Result<V, E>`
  so closures work directly with `optimize`
- Rewrite optimize, optimize_async, optimize_parallel to accept
  `impl Objective<V>` with before_trial/after_trial hooks
- Remove optimize_with, optimize_with_async, optimize_with_parallel
- Remove max_retries and retry logic from Objective trait
- Add explicit closure type annotations for HRTB inference
- Convert FnMut test closures to Fn via RefCell/Cell

examples/early_stopping.rs

@@ -44,7 +44,7 @@ fn main() -> optimizer::Result<()> {
         target: 0.01,
     };
 
-    study.optimize_with(100, objective)?;
+    study.optimize(100, objective)?;
 
     let best = study.best_trial()?;
     println!(