`Deref` in `Symbol` can leak a raw function pointer.

[lvella]

I can just take a Symbol, deref into a &T, clone the function pointer T, and boom, I am completely free from the 'lib lifetime.

Considering function pointers always have 'static lifetime, I don't think there is any safe alternative for this problem. Maybe manually implement Fn trait? But this is unstable.

[nagisa]

Duplicate of #13