[board, position] Add a color argument to opposing_sight

New convention: active_color_ methods operate on the active color of the Board.
Methods without that prefix take a color parameter and operate on that.

Refactor opposing_sight to do this.

position/src/position/position.rs

@@ -81,8 +81,8 @@ impl Position {
     }
 
     /// A [`BitBoard`] of all squares visible by colors that oppose the given color.
-    pub fn opposing_sight(&self) -> BitBoard {
-        self.board.opposing_sight()
+    pub fn active_color_opposing_sight(&self) -> BitBoard {
+        self.board.active_color_opposing_sight()
     }
 }