consider making `Signal: Clone`

[databasedav]

it's annoying to have to specify + Clone in function signatures where a generic signal needs to be cloned, while this has the benefit of being rustic in its minimalism and having to specify the intention to clone up front, it both has ergonomic benefits and all concrete Signal types are already Clone

in the meantime, one can specify a custom trait e.g.

pub trait SignalClone: Signal + Clone {}

fn f(s: impl SignalClone<Item = i32>) {
    s.clone();
    ...
}