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/lib.rs

@@ -26,7 +26,7 @@
 //! let x = FloatParam::new(-10.0, 10.0).name("x");
 //!
 //! study
-//!     .optimize(50, |trial| {
+//!     .optimize(50, |trial: &mut optimizer::Trial| {
 //!         let v = x.suggest(trial)?;
 //!         Ok::<_, Error>((v - 3.0).powi(2))
 //!     })