[explorer, moves, core] Improve error handling in explorer

Implement thiserror::Error for a bunch of error types, and remove string errors
from the implementation of the command handler in explorer.

Clean up parsing of basic types all over the place.

Update Cargo files to include thiserror and anyhow.

core/src/lib.rs

@@ -2,11 +2,12 @@
 
 pub mod colors;
 pub mod coordinates;
-pub mod errors;
 pub mod pieces;
+pub mod shapes;
 
 mod macros;
 
 pub use colors::Color;
 pub use coordinates::{Direction, File, Rank, Square};
-pub use pieces::{Piece, PlacedPiece, Shape};
+pub use pieces::{Piece, PlacedPiece};
+pub use shapes::Shape;