Fix Exclude & Lead to dispose enumerator

This is a squashed merge or PR #673 that closes #649.

It fixes mistakes made in commit "Replace using statements with
declarations" (hash cdc6f76)
as part of PR #622.

MoreLinq/Exclude.cs

@@ -45,7 +45,7 @@ public static IEnumerable<T> Exclude<T>(this IEnumerable<T> sequence, int startI
             {
                 var index = -1;
                 var endIndex = startIndex + count;
-                var iter = sequence.GetEnumerator();
+                using var iter = sequence.GetEnumerator();
                 // yield the first part of the sequence
                 while (iter.MoveNext() && ++index < startIndex)
                     yield return iter.Current;

MoreLinq/Lead.cs

@@ -65,7 +65,7 @@ public static IEnumerable<TResult> Lead<TSource, TResult>(this IEnumerable<TSour
             return _(); IEnumerable<TResult> _()
             {
                 var leadQueue = new Queue<TSource>(offset);
-                var iter = source.GetEnumerator();
+                using var iter = source.GetEnumerator();
 
                 bool hasMore;
                 // first, prefetch and populate the lead queue with the next step of