Derive Copy trait

- We can now derive Copy trait because RangeInclusive no longer implements Iterator, just IntoIterator. Discussions on not deriving Copy made the argument that it doesn't remain clear when an iterator is mutating the range or not (Iterator and Copy should be mutually exclusive?)
see issue: rust-lang/rfcs#2848

- also updated is_empty with the defn found in the std library.

src/lib.rs

@@ -4,7 +4,7 @@
 use core::iter::Step;
 use core::ops::{RangeBounds, Bound, Bound::Included};
 
-#[derive(Debug, Clone, PartialEq, Eq)]
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
 pub struct RangeInclusive<Idx: Clone + PartialOrd> {
     pub(crate) start: Idx,
     pub(crate) end: Idx
@@ -36,7 +36,7 @@ impl<Idx: Clone + PartialOrd> RangeInclusive<Idx> {
     }
 
     pub fn is_empty(&self) -> bool {
-        self.end < self.start
+        !(self.start <= self.end)
     }
 
     pub fn contains<U>(&self, item: &U) -> bool
@@ -82,4 +82,4 @@ impl<A: Step> Iterator for RangeInclusiveIterator<A> {
             Some(core::mem::replace(&mut self.offset, n))
         }
     }
-}
+}