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

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

core/src/lib.rs

@@ -1,3 +1,12 @@
-mod coordinates;
+// Eryn Wells <eryn@erynwells.me>
 
+pub mod colors;
+pub mod coordinates;
+pub mod errors;
+pub mod pieces;
+
+mod macros;
+
+pub use colors::Color;
 pub use coordinates::{Direction, File, Rank, Square};
+pub use pieces::{Piece, PlacedPiece, Shape};