feat: add parameter importance via Spearman rank correlation

Compute per-parameter importance scores by measuring the absolute
Spearman rank correlation between each parameter's values and the
objective across completed trials. Scores are normalized to sum to 1.0
and returned sorted by descending importance.

src/lib.rs

@@ -216,6 +216,7 @@ macro_rules! trace_debug {
 
 mod distribution;
 mod error;
+mod importance;
 mod kde;
 mod param;
 pub mod parameter;