Implement AddAssign and friends for Signal<T> where T: Copy

[lukechu10]

This makes it more ergonomic for simple cases where we want to increment/decrement a Signal.
e.g. instead of writing this:

counter.set(*counter.get() + 1);

we can write this:

counter += 1;

This should only be implemented for types where T: Copy because we don't want to implicitly clone values behind the scenes.