Implement signal.into_inner()

[stefnotch]

This implements into_inner the way it is implemented for Arcs. This fixes #3068 .

However, with the non-arc signals, the behaviour can be rather unintuitive. For example, the following does not succeed.
Why?
Because there's still the original RwSignal that was created inside the signal function. (RwSignal::new(value).split() creates 3 signals)

#[test]
fn into_inner_non_arc_signal() {
    let owner = Owner::new();
    owner.set();

    let (a, b) = signal(2);
    assert_eq!(a.get(), 2);
    b.dispose(); // strong count is now 2
    assert_eq!(a.into_inner(), Some(2)); // fails
}

I'm happy to document this, or to attempt a fix, or to remove the trait for normal signals.

[gbj]

I think my changes here to the implementation of signal() fix this, and cause the signal() test to work. It initially failed because signal() was implemented as RwSignal::new(value).split(), which means there's another reference to the inner ArcRwSignal (because it created an RwSignal and stored it in the owner). This version should be strictly better, in that it means only allocating 2 slots in the arena per signal() instead of 3, which is better for memory anyway. Thanks for surfacing that with the test, and for implementing this useful trait!

[stefnotch]

Sweet, thank you for bringing this over the finish line.