Return a chessfriend_moves::EnPassant from a new Position::en_passant()

Replace Position's en_passant_target_square() and en_passant_capture_square()
with a single en_passant() method that returns a new EnPassant struct that has
the target and capture squares for the en passant move.

moves/src/en_passant.rs

@@ -0,0 +1,49 @@
+// Eryn Wells <eryn@erynwells.me>
+
+use chessfriend_core::{Rank, Square};
+
+/// En passant information.
+#[derive(Clone, Copy, Debug)]
+pub struct EnPassant {
+    target: Square,
+    capture: Square,
+}
+
+impl EnPassant {
+    fn _capture_square(target: Square) -> Option<Square> {
+        let (file, rank) = target.file_rank();
+        match rank {
+            Rank::THREE => Some(Square::from_file_rank(file, Rank::FOUR)),
+            Rank::SIX => Some(Square::from_file_rank(file, Rank::FIVE)),
+            _ => None,
+        }
+    }
+
+    /// Return en passant information for a particular target square. The target
+    /// square is the square a pawn capturing en passant will move to.
+    ///
+    /// Return `None` if the square is not eligible for en passant.
+    ///
+    /// ## Examples
+    ///
+    /// ```
+    /// use chessfriend_core::Square;
+    /// use chessfriend_moves::EnPassant;
+    /// assert!(EnPassant::from_target_square(Square::E3).is_some());
+    /// assert!(EnPassant::from_target_square(Square::B4).is_none());
+    /// ```
+    pub fn from_target_square(target: Square) -> Option<Self> {
+        match Self::_capture_square(target) {
+            Some(capture) => Some(Self { target, capture }),
+            None => None,
+        }
+    }
+
+    pub fn target_square(&self) -> Square {
+        self.target
+    }
+
+    pub fn capture_square(&self) -> Square {
+        self.capture
+    }
+}

moves/src/lib.rs

@@ -3,9 +3,11 @@
 mod builder;
 mod castle;
 mod defs;
+mod en_passant;
 mod moves;
 
 pub use builder::{Builder, Error as BuilderError};
 pub use castle::Castle;
 pub use defs::PromotionShape;
+pub use en_passant::EnPassant;
 pub use moves::Move;