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/fanova.rs

@@ -41,7 +41,7 @@
 //! let y = FloatParam::new(0.0, 10.0).name("y");
 //!
 //! study
-//!     .optimize(50, |trial| {
+//!     .optimize(50, |trial: &mut optimizer::Trial| {
 //!         let xv = x.suggest(trial)?;
 //!         let yv = y.suggest(trial)?;
 //!         // x matters much more than y