[core,board] Move board::piece to core

Break up types in core into finer grained modules.
Update all the imports.

board/src/move_generator/move_set.rs

@@ -1,5 +1,8 @@
-use crate::{piece::PlacedPiece, Move};
+// Eryn Wells <eryn@erynwells.me>
+
+use crate::Move;
 use chessfriend_bitboard::BitBoard;
+use chessfriend_core::PlacedPiece;
 
 #[derive(Clone, Debug, Eq, PartialEq)]
 struct BitBoardSet {