Remove BitBoardBuilder

It's unused except for the macro, and BitBoard itself can be declared mutable,
and implements Copy and Clone. So, I don't think having a separate Builder type
helps much.

bitboard/src/lib.rs

@@ -5,15 +5,17 @@ mod bitboard;
 mod library;
 mod shifts;
 
-pub use bitboard::{BitBoard, BitBoardBuilder};
+pub use bitboard::BitBoard;
 
 pub(crate) use bit_scanner::{LeadingBitScanner, TrailingBitScanner};
 
 #[macro_export]
 macro_rules! bitboard {
-    ($($sq:ident),* $(,)?) => {
-        $crate::BitBoardBuilder::empty()
-            $(.square(chessfriend_core::Square::$sq))*
-            .build()
+    ($($sq:ident)* $(,)?) => {
+        {
+            let mut bitboard = $crate::BitBoard::empty();
+            $(bitboard.set(chessfriend_core::Square::$sq);)*
+            bitboard
+        }
     };
 }