feat(traits): add Initializer, Variation, and Optimizer traits

The three operator-level traits the algorithms consume, plus the single
trait users implement to add a new optimizer (`Optimizer<P>`). All take
`&mut Rng` directly rather than being generic over the RNG (spec §7.8).

src/traits/mod.rs

@@ -0,0 +1,9 @@
+//! The small set of traits that user code and built-in algorithms implement.
+
+pub mod initializer;
+pub mod optimizer;
+pub mod variation;
+
+pub use initializer::*;
+pub use optimizer::*;
+pub use variation::*;