feat(operators): add BitFlipMutation for Vec<bool>

Variation that clones the first parent and flips each bit independently
with probability `probability`. Panics if probability is outside [0, 1]
(spec §11.3).

Tests verify that probability=0 produces an unchanged child and
probability=1 flips every bit (spec §18.3).

src/prelude.rs

@@ -16,4 +16,4 @@ pub use crate::pareto::{
     pareto_compare, pareto_front,
 };
 
-pub use crate::operators::{GaussianMutation, RealBounds};
+pub use crate::operators::{BitFlipMutation, GaussianMutation, RealBounds};