Reorganize board code in engine

Just a bit

rs/src/board.rs

@@ -242,6 +242,11 @@ pub struct Board {
 }
 
 impl Board {
+    pub fn new() -> Board {
+        Board {
+            state: Default::default(),
+        }
+    }
     fn get_at(&self, position: &Position) -> Option<&Piece> {
         self.state.get(position)
     }
@@ -260,11 +265,8 @@ impl Board {
         self.set_at(target, Some(piece));
         Ok(())
     }
-
-    pub fn new() -> Board {
-        Board {
-            state: Default::default(),
-        }
+    pub fn clear(&mut self) {
+        self.state.clear();
     }
     pub fn reset(&mut self) {
         self.state.clear();