key-value logging

[yoshuawuyts]

Now that experimental key-value logging has landed in log (rust-lang/log#328), we should look into using it for async-log.

This is currently blocked on having a way to actually log key-value entries, as the macros in log don't support this yet. I've asked a question about this in rust-lang/log#328 (comment), which hopefully lead to a resolution where we can start logging key-value pairs!

[yoshuawuyts]

This mostly works now because of #5. The only piece missing is adding support to the #[instrument] macro.