Custom Cell implementation is unsound

[Shnatsel]

Right now there is no mechanism in Cell to track whether a mutable reference to the data is already acquired. Thus it is possible to obtain several mutable references to the same memory location by calling Cell::get_mut() repeatedly:

let mycell = Cell::new(vec![1,2,3]);
let ref1 = mysell.get_mut();
let ref2 = mysell.get_mut(); // obtained a second mutable reference; UB starts here

This may result in pretty much arbitrary memory corruption, most likely a use-after-free. Even though no code internal to Actix makes two obvious calls to get_mut() in a row, this behavior has been shown to be exploitable from the public API (see PoC that fails MIRI).

PR #158 has removed one instance of this Cell and all uses of it. However, there is another copy of it in the repository in actix-utils crate, and there are 23 calls to .get_mut().

The obvious fix is to replace all uses of custom Cell<T> with Rc<RefCell<T>> (the way it was before the introduction of a custom cell in 20b03a4), like it was done in #158.