Handle::remove_signal

[vorner]

The iterator::backend::Handle has add_signal, but no remove_signal. This is a bit inconsistent and it turns out people would find it useful (see #63).

By implementing it on Handle, one would get the remove_signal for all the async structures's handles and the base iterator one. It still needs exposing on each of these types separately as a method wrapper (the signals.handle().remove_signal gets „propagated“ as the Handle type is the same one for all of them, but signals.remove_signal needs to actually be added).