Make the ref hook a proper ref

[glennsl]

This has the ref hook return an actual ref, removes the setter, and therefore acts, and has to be used, like an ordinary ref that persists across calls.

let%hook count = Hooks.ref(0);
count := count^ + 1; // Instead of `setCount(count + 1)`

The rationale for this change is that the name ref implies that the value 1) will be updated immediately, and 2) is a reference that will change its contents when updated elsewhere. Neither of which is true. It therefore acts like a bit of a trap which some people (i.e. me) fall into multiple times despite knowing how it actually works. An example is in revery-ui/revery#648. This change makes both of these implications true, while still fulfilling the role of the current ref hook, but at the cost of using a slightly different API than both React and the state hook.

[wokalski]

Thank you @glennsl. It's a very welcome addition.

[glennsl]

Thank you for the very quick acceptance and response!