refactor: move all type re-exports out of crate root into their modules

- Sampler, pruner, storage, parameter, and multi_objective types are no
  longer re-exported at the crate root; access via module paths instead
  (e.g. optimizer::sampler::TpeSampler, optimizer::parameter::FloatParam)
- Prelude continues to re-export everything for convenience
- Add module-level pub use re-exports in sampler/mod.rs and parameter.rs
- Update derive macro to reference optimizer::parameter::Categorical
- Rename sampler::differential_evolution to sampler::de
- Fix all downstream imports in tests, benches, and doctests
- Fix all rustdoc intra-doc links to use explicit module paths

src/study.rs

@@ -382,8 +382,8 @@ where
     /// ```
     /// use std::collections::HashMap;
     ///
-    /// use optimizer::parameter::{FloatParam, IntParam, Parameter};
-    /// use optimizer::{Direction, ParamValue, Study};
+    /// use optimizer::parameter::{FloatParam, IntParam, ParamValue, Parameter};
+    /// use optimizer::{Direction, Study};
     ///
     /// let study: Study<f64> = Study::new(Direction::Minimize);
     /// let x = FloatParam::new(0.0, 10.0);