Add MappedAtomicRef(Mut)

[LechintanTudor]

Implements the functionality described here #11

[Imberflur]

'b is the lifetime of the shared borrow of the AtomicRefCell (and not actually the lifetime that &T is valid for which is up until the AtomicRef/MappedAtomicRef is dropped).

Unfortunately, as noted here rust-lang/rust#70263 (comment), that means this API allows for UB.

E.g. the following test compiles:

#[test]
fn owned_map() {
    use atomic_refcell::*;

    let cell = AtomicRefCell::new(3u8);
    let mut other = &mut 0u8;

    let guard = AtomicRefMut::map_into(cell.borrow_mut(), |inner: &mut u8| {
        // sneak reference out
        other = inner;
    });
    // guard dropped but reference still accessible via `other`
    drop(guard);

    assert_eq!(*other, 3);

    *cell.borrow_mut() = 1;

    assert_eq!(*other, 1);
}

There are some potential workarounds noted in the comment I linked, but they can make things difficult to work with.