Figure out a much better API for dedupe_map

Right now this is the dedupe_map API:

```rust
signal.dedupe_map(move |new_value: &mut A| -> B {
    *new_value
})
```

But the user should be able to determine three things:

1. Whether the old value is equal to the new value or not.
2. What the new output value should be.
3. What the old stored value should be.