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/journal_storage.rs

@@ -24,7 +24,7 @@ fn main() -> optimizer::Result<()> {
             .storage(storage)
             .build();
 
-        study.optimize(20, |trial| {
+        study.optimize(20, |trial: &mut optimizer::Trial| {
             let xv = x.suggest(trial)?;
             Ok::<_, optimizer::Error>(xv * xv)
         })?;
@@ -46,7 +46,7 @@ fn main() -> optimizer::Result<()> {
             .build();
 
         let before = study.n_trials();
-        study.optimize(10, |trial| {
+        study.optimize(10, |trial: &mut optimizer::Trial| {
             let xv = x.suggest(trial)?;
             Ok::<_, optimizer::Error>(xv * xv)
         })?;