fix(permutations-heap): do the swaps on a copy of the input

Heap's Algorithm generates each permutation by swapping two elements of a
working array in place, and that working array was the argument itself, so
permutationsHeap([1, 2, 3]) left the caller holding [3, 2, 1]. Two calls on
one array returned different lists, and a frozen array threw.

Copy the input up front and swap on the copy, as shuffle and numericSort
already do. The returned permutations are unchanged.

test/permutations_heap.test.js

@@ -16,4 +16,19 @@ describe("permutationsHeap", function () {
             [3, 2, 1]
         ]);
     });
+    it("does not modify the caller's array", function () {
+        const x = [1, 2, 3];
+        ss.permutationsHeap(x);
+        assert.deepEqual(x, [1, 2, 3]);
+    });
+    it("accepts a frozen array", function () {
+        assert.deepEqual(ss.permutationsHeap(Object.freeze([1, 2])), [
+            [1, 2],
+            [2, 1]
+        ]);
+    });
+    it("returns the same permutations when called twice on one array", function () {
+        const x = [1, 2, 3];
+        assert.deepEqual(ss.permutationsHeap(x), ss.permutationsHeap(x));
+    });
 });