Mark XY::stack_{horizontal, vertical} as must_use (#511)

The method names stack_horizontal and stack_vertical don’t make it clear
whether the methods modify self or return the modified version.
Therefore, it is easy to use them wrong if you don’t look at the
documentation.  This patch adds the must_use attribute to both methods
to make it easier to spot such mistakes.

cursive-core/src/vec.rs

@@ -258,6 +258,7 @@ impl<T: Ord> XY<T> {
 
 impl<T: Ord + Add<Output = T> + Clone> XY<T> {
     /// Returns (max(self.x,other.x), self.y+other.y)
+    #[must_use]
     pub fn stack_vertical(&self, other: &Self) -> Self {
         Self::new(
             max(self.x.clone(), other.x.clone()),
@@ -266,6 +267,7 @@ impl<T: Ord + Add<Output = T> + Clone> XY<T> {
     }
 
     /// Returns (self.x+other.x, max(self.y,other.y))
+    #[must_use]
     pub fn stack_horizontal(&self, other: &Self) -> Self {
         Self::new(
             self.x.clone() + other.x.clone(),