[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/pieces/display.rs

@@ -0,0 +1,28 @@
+// Eryn Wells <eryn@erynwells.me>
+
+use super::Piece;
+
+#[derive(Default)]
+pub enum PieceDisplayStyle {
+    #[default]
+    Unicode,
+    ASCII,
+    LongForm,
+}
+
+pub struct PieceDisplay {
+    pub(super) piece: Piece,
+    pub(super) style: PieceDisplayStyle,
+}
+
+impl std::fmt::Display for PieceDisplay {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self.style {
+            PieceDisplayStyle::Unicode => write!(f, "{}", self.piece.to_unicode()),
+            PieceDisplayStyle::ASCII => write!(f, "{}", self.piece.to_ascii()),
+            PieceDisplayStyle::LongForm => {
+                write!(f, "{} {}", self.piece.color.name(), self.piece.shape.name())
+            }
+        }
+    }
+}