Derive Default for XY
This patch derives Default for XY<T>.  This makes it easier to deal with
types like XY<usize>.
@@ -2,7 +2,7 @@ use crate::direction::Orientation;
use std::iter;
/// A generic structure with a value for each axis.
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+#[derive(Debug, Default, Clone, Copy, PartialEq, Eq, Hash)]
pub struct XY<T> {
/// X-axis value
pub x: T,
