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

test/io/write.wren

@@ -0,0 +1,12 @@
+class Foo {
+  toString { return "Foo.toString" }
+}
+
+// Calls toString on argument.
+IO.write(new Foo) // expect: Foo.toString
+
+// Returns argument.
+var result = IO.write(123) // expect: 123
+IO.write(result == 123) // expect: true
+
+// TODO: What if toString on argument doesn't return a string?