Mark instrumented functions

[Invizory]

We should mark transformed functions in some way to properly identify instrumented functions at runtime, for example, via Reflection.isInstrumented(function).

This feature will be useful when deciding if value should remain tainted when have passed as an argument to some function or not.

Also, there is a bug in the current stub for isInstrumented:

import {reflection} from "taintflow-runtime";

function foo() { }
reflection.isInstrumented(foo); // => true, ok
reflection.isInstrumented(foo.bind(this)); // => false, ???