Make IO.write() call toString on its argument.

test/list/to_string.wren

@@ -0,0 +1,17 @@
+// Handle empty list.
+IO.write([].toString)             // expect: []
+
+// Does not quote strings.
+IO.write([1, "2", true].toString) // expect: [1, 2, true]
+
+// Nested lists.
+IO.write([1, [2, [3], 4], 5]) // expect: [1, [2, [3], 4], 5]
+
+// Calls toString on elements.
+class Foo {
+  toString { return "Foo.toString" }
+}
+
+IO.write([1, new Foo, 2]) // expect: [1, Foo.toString, 2]
+
+// TODO: Handle lists that contain themselves.