Make tor-rtcompat traits sealed in 1.0

We've realized today that, although tor-rtcompat is an important piece of our public API, we do not currently plan to declare it 1.0. That is, we need to make sure that we can continue to improve its feature set even as we keep arti_client stable.

To do this, we plan to make the traits in tor-rtcompat sealed by default for now, with an optional unseal-traits feature to unseal them.

@Diziet has taken this on.