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

src/visualization.rs

@@ -26,7 +26,7 @@
 //!
 //! let study: Study<f64> = Study::new(Direction::Minimize);
 //! # let x = FloatParam::new(0.0, 1.0);
-//! # study.optimize(10, |trial| {
+//! # study.optimize(10, |trial: &mut optimizer::Trial| {
 //! #     let v = x.suggest(trial)?;
 //! #     Ok::<_, optimizer::Error>(v * v)
 //! # }).unwrap();