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

tests/study/summary.rs

@@ -8,7 +8,7 @@ fn test_summary_with_completed_trials() {
     let x = FloatParam::new(0.0, 10.0).name("x");
 
     study
-        .optimize(5, |trial| {
+        .optimize(5, |trial: &mut optimizer::Trial| {
             let val = x.suggest(trial)?;
             Ok::<_, Error>(val * val)
         })
@@ -61,7 +61,7 @@ fn test_display_matches_summary() {
     let x = FloatParam::new(0.0, 10.0).name("x");
 
     study
-        .optimize(3, |trial| {
+        .optimize(3, |trial: &mut optimizer::Trial| {
             let val = x.suggest(trial)?;
             Ok::<_, Error>(val)
         })