[moves] Implement promotions and en passant in the PawnMoveGenerator

Add another sub-iterator to the PawnMoveGenerator that produces promotion pushes
or capture moves (depending on move_type) when a pawn moves to the back rank.

Also implement en passant moves.

Fix a bug in the Left and Right captures branches where the wrong neighbors used
to calculate origin squares.

Add a whole bunch of tests. Still missing several cases though.

moves/src/generators.rs

@@ -6,7 +6,13 @@ pub use pawn::PawnMoveGenerator;
 
 use crate::Move;
 
-#[derive(Clone, Debug, Eq, PartialEq)]
+#[derive(Clone, Debug, Eq, Hash, PartialEq)]
 pub struct GeneratedMove {
     pub(crate) ply: Move,
 }
+
+impl From<Move> for GeneratedMove {
+    fn from(value: Move) -> Self {
+        GeneratedMove { ply: value }
+    }
+}