Nice function execution API

[chfast]

I think using methods instead of free function may be nicer and will be more JavaScript-like.

In particular, for function execution we may have something like:

auto instance = instantiate();

auto fn = instance->fn("func1");
fn(0, 1.0);

instance->fn("func2")(1, 2);

instance->fn(0)(-0.0f);   // Get function by index.

[axic]

Why is it important to be "Javascript-like"?