The Rust community appear to have converged on tracing - it's used by
the Rust compiler, and receives close to 10x the number of downloads
that slog does. Its API is more ergonomic, and it does a much nicer
job with async rust.
To make this change, we no longer pass around explicit loggers, and rely
on a globally-registered one. The log file location has been changed
from one in each profile folder to a single one in the base folder. This
will remain empty for most users, since only errors are logged by default,
but may be useful for debugging future changes.