feat: add prelude module

A single `use heuropt::prelude::*;` brings in the common user-facing
types and traits available so far. Subsequent commits add Pareto
helpers, operators, and algorithms to the same prelude as they land.

src/lib.rs

@@ -3,4 +3,5 @@
 //! full design.
 
 pub mod core;
+pub mod prelude;
 pub mod traits;