Passing tainted value to native functions in the object

[Invizory]

Examples:

const foo = {};
Reflect.defineProperty(foo, "bar", {
    get: Flow.tainted(() => "bar"),
});
// TypeError: Getter must be a function: [object Object]

JSON.stringify({foo: Flow.tainted("bar")}); 

Caveat: if we consider resolving this problem by deep copy, we should also find the way to preserve semantics of functions like Object.freeze(obj).

Related to: #48.