Test the various heuristics of erase_indices

This is a targeted test to make sure we cover all the heuristic edge
cases in `erase_indices`, used by `drain` and other methods.

I found a failure from `cargo mutants` where we still passed tests after
`erase_indices_sweep` was replaced with an empty body. I was concerned
because that function contains `unsafe` code, so we *really* need it
tested. It turns out that we do *sometimes* hit that in `quickcheck`
tests, but might miss that if we're randomly unlucky, so this PR adds a
new test that will hit all the edge cases every time.

src/map/tests.rs

@@ -541,6 +541,26 @@ fn into_values() {
     assert!(values.contains(&'c'));
 }
 
+#[test]
+fn drain_range() {
+    // Test the various heuristics of `erase_indices`
+    for range in [
+        0..0,   // nothing erased
+        10..90, // reinsert the few kept (..10 and 90..)
+        80..90, // update the few to adjust (80..)
+        20..30, // sweep everything
+    ] {
+        let mut vec = Vec::from_iter(0..100);
+        let mut map: IndexMap<i32, ()> = (0..100).map(|i| (i, ())).collect();
+        drop(vec.drain(range.clone()));
+        drop(map.drain(range));
+        assert!(vec.iter().eq(map.keys()));
+        for (i, x) in vec.iter().enumerate() {
+            assert_eq!(map.get_index_of(x), Some(i));
+        }
+    }
+}
+
 #[test]
 #[cfg(feature = "std")]
 fn from_array() {