[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::*;

board/src/bitboard/mod.rs

@@ -4,4 +4,13 @@ mod library;
 mod shifts;
 
 pub(crate) use bit_scanner::{LeadingBitScanner, TrailingBitScanner};
-pub(crate) use bitboard::BitBoard;
+pub(crate) use bitboard::{BitBoard, BitBoardBuilder};
+
+#[macro_export]
+macro_rules! bitboard {
+    ($($sq:ident),*) => {
+        $crate::bitboard::BitBoardBuilder::empty()
+            $(.square($crate::Square::$sq))*
+            .build()
+    };
+}

board/src/lib.rs

@@ -1,5 +1,6 @@
 // Eryn Wells <eryn@erynwells.me>
 
+#[macro_use]
 mod bitboard;
 mod display;
 mod moves;