Add isEmpty to Sequence.

Thanks, Michel and Thorbjørn!

test/core/sequence/is_empty.wren

@@ -0,0 +1,10 @@
+IO.print([].isEmpty) // expect: true
+IO.print([1].isEmpty) // expect: false
+
+class InfiniteSequence is Sequence {
+  iterate(iterator) { true }
+  iteratorValue(iterator) { iterator }
+}
+
+// Should not try to iterate the whole sequence.
+IO.print((new InfiniteSequence).isEmpty) // expect: false