Simplify `log_level()`

[hadley]

Previously created a function then immediately called it. Now it just inlines the results of that function, which leads to a bunch of simplification. I think this changes suggests that we could deprecate logger() since I doubt there's much use for it outside the package.

This is technically a breaking change since it alters the return value of log_level() and friends. But the current return value was not documented, and it seems unlikely that folks would be dependent on the specific value. Still needs a NEWS bullet though.