Exploring if app-services can replace log with tracing.

app is a pretend app, much like gecko, firefox-android or firefox-ios.

second_lib is a utility crate, called by app.

first_lib is another utility crate, called by second_lib

log_lib is another utility crate, but one which still used the log crate.

All crates other than log_lib use tracing rather than log. Further, app has a "tracing subscriber" which is used to ferry logs across an FFI.