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/basic_optimization.rs

@@ -17,7 +17,7 @@ fn main() {
 
     // Run 50 trials, each evaluating f(x) = (x - 3)²
     study
-        .optimize(50, |trial| {
+        .optimize(50, |trial: &mut optimizer::Trial| {
             let x_val = x.suggest(trial)?;
             let value = (x_val - 3.0).powi(2);
             Ok::<_, Error>(value)