Macro hygiene of chain!

If someone has a module named `core` and uses `itertools::chain!` then there is a conflict!
I improve our macro hygiene test by adding (empty) modules with all the names we might be tempted to use.
Without changing `core` in `chain!`, the test failed!

src/lib.rs

@@ -393,16 +393,16 @@ macro_rules! izip {
 /// ```
 macro_rules! chain {
     () => {
-        core::iter::empty()
+        $crate::__std_iter::empty()
     };
     ($first:expr $(, $rest:expr )* $(,)?) => {
         {
-            let iter = core::iter::IntoIterator::into_iter($first);
+            let iter = $crate::__std_iter::IntoIterator::into_iter($first);
             $(
                 let iter =
-                    core::iter::Iterator::chain(
+                    $crate::__std_iter::Iterator::chain(
                         iter,
-                        core::iter::IntoIterator::into_iter($rest));
+                        $crate::__std_iter::IntoIterator::into_iter($rest));
             )*
             iter
         }

tests/macros_hygiene.rs

@@ -1,3 +1,8 @@
+mod alloc {}
+mod core {}
+mod either {}
+mod std {}
+
 #[test]
 fn iproduct_hygiene() {
     let _ = itertools::iproduct!();
@@ -12,3 +17,11 @@ fn izip_hygiene() {
     let _ = itertools::izip!(0..6, 0..9);
     let _ = itertools::izip!(0..6, 0..9, 0..12);
 }
+
+#[test]
+fn chain_hygiene() {
+    let _: ::std::iter::Empty<i32> = itertools::chain!();
+    let _ = itertools::chain!(0..6);
+    let _ = itertools::chain!(0..6, 0..9);
+    let _ = itertools::chain!(0..6, 0..9, 0..12);
+}