[core] Fix SLF4J log levels

[hearnadam]

I noticed that only error was enabled for my Kyo app, when I had it set to info. Info logs should have shown up, but they didn't.

Solution

Check the level of log level enabled in reverse order. In the previous case, error was always enabled (unless off).

Notes

• I changed Level to a sealed abstract class, enabling exhaustive pattern matching. Avoiding AnyVal is generally better if we know we will only have static instances.
• I also added a CanEqual for users.

Checklist

• Unit test all changes
• Update scaladocs if needed
• Update the README if needed