Add count to Sequence.

test/sequence/count.wren

@@ -0,0 +1,11 @@
+class TestSequence is Sequence {
+  iterate(iterator) {
+    if (iterator == null) return 1
+    if (iterator == 10) return false
+    return iterator + 1
+  }
+
+  iteratorValue(iterator) { iterator }
+}
+
+IO.print((new TestSequence).count) // expect: 10