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

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

core/src/errors.rs

@@ -0,0 +1,7 @@
+// Eryn Wells <eryn@erynwells.me>
+
+#[derive(Debug, Eq, PartialEq)]
+pub struct TryFromCharError;
+
+#[derive(Debug, Eq, PartialEq)]
+pub struct TryFromStrError;