feat: add serde serialization support behind serde feature flag

Add Serialize/Deserialize derives to all public data types (ParamValue,
Distribution, Direction, TrialState, ParamId, AttrValue, CompletedTrial)
gated behind a `serde` feature flag. Introduce StudySnapshot struct and
Study::save()/Study::load() for persisting and restoring study state as
human-readable JSON.

src/trial.rs

@@ -15,6 +15,7 @@ use crate::types::TrialState;
 
 /// A user attribute value that can be stored on a trial.
 #[derive(Clone, Debug, PartialEq)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub enum AttrValue {
     /// A floating-point attribute.
     Float(f64),