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;

src/prelude.rs

@@ -0,0 +1,12 @@
+//! Common imports for users of `heuropt`.
+//!
+//! ```
+//! use heuropt::prelude::*;
+//! ```
+
+pub use crate::core::{
+    Candidate, Direction, Evaluation, Objective, ObjectiveSpace, OptimizationResult,
+    Population, Problem, Rng, rng_from_seed,
+};
+
+pub use crate::traits::{Initializer, Optimizer, Variation};