Re-introduce SL4J logging interfaces.

As mentioned in !31 (comment 3249526), without these I get the following warnings:

SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.

The tests don't show this warning, so I'd have to do some work to figure out how to reproduce the issue in CI.

merged

mentioned in commit a58e3d46