[board] Declare three new Display-like traits

- ASCIIDisplay → format a type using ASCII only characters
- UnicodeDisplay → format a type using any Unicode characters
- FENDisplay → format a type for inclusion in a FEN string

board/src/display.rs

@@ -0,0 +1,15 @@
+// Eryn Wells <eryn@erynwells.me>
+
+use std::fmt;
+
+pub(crate) trait ASCIIDisplay {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result;
+}
+
+pub(crate) trait UnicodeDisplay {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result;
+}
+
+pub(crate) trait FENDisplay {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result;
+}