Move some functionality into the new Engine mod

This will help with separating the "thinking" logic from the move rules
and stuff.

rs/src/engine.rs

@@ -0,0 +1,43 @@
+use crate::board;
+
+pub struct Engine {
+    board: board::Board,
+}
+
+impl Engine {
+    pub fn new() -> Self {
+        let mut board = board::Board::new();
+        board.reset();
+        Engine {
+            board,
+        }
+    }
+    pub fn get_state(&self) -> &board::Board {
+        &self.board
+    }
+    pub fn set_state(&mut self, board: board::Board) {
+        self.board = board;
+    }
+    pub fn get_legal_moves(
+        &self,
+        position: &board::Position,
+    ) -> Result<Vec<board::Position>, ()> {
+        Ok(self
+            .board
+            .find_moves(position)?
+            .chain(self.board.find_captures(position)?)
+            .collect())
+    }
+    pub fn make_move(
+        &mut self,
+        source: &board::Position,
+        target: board::Position,
+    ) -> Result<(), ()> {
+        if !self.get_legal_moves(source)?.contains(&target) {
+            Err(())
+        } else {
+            // We checked that there is a piece at source in get_legal_moves
+            self.board.relocate(source, target)
+        }
+    }
+}