Consistency between various *default methods

[jonhoo]

Feature Request

• tracing::dispatcher::set_default takes &Dispatch
• tracing::dispatcher::set_global_default takes Dispatch
• tracing::dispatcher::with_default takes &Dispatch

These should all take one or the other. &Dispatch is definitely more ergonomic (and we clone internally), Dispatch saves us a clone in certain cases. Either way, we should stick with one.

[hawkw]

Unfortunately, I think this is a breaking change. So, we can do it, but it'll have to wait until the next time we break APIs in tracing-core (which may not be for a while)...