[board] Create a bitboard! macro and BitBoardBuilder

The Builder enables cleanly building a BitBoard out of squares.
The macro lets you create a BitBoard from a simple list of coordinates:

bitboard!(A1, B2, C3, D4, …)

board/src/bitboard/bitboard.rs

@@ -188,6 +188,27 @@ impl Not for &BitBoard {
     }
 }
 
+pub struct BitBoardBuilder(BitBoard);
+
+impl BitBoardBuilder {
+    pub const fn empty() -> BitBoardBuilder {
+        BitBoardBuilder(BitBoard::empty())
+    }
+
+    pub fn new(bits: u64) -> BitBoardBuilder {
+        BitBoardBuilder(BitBoard::new(bits))
+    }
+
+    pub fn square(mut self, square: Square) -> BitBoardBuilder {
+        self.0.set_square(square);
+        self
+    }
+
+    pub fn build(&self) -> BitBoard {
+        self.0
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;