Convert RulesLog to a logging service

Drupal 7 contained many rules_log() calls from the rules plugins / rules expression classes, which were responsible for keeping the rules debug log and showing it in case of evaluation errors.

In Drupal 8, we want to do so similar but use a logger service instead of the function. As a first step, we can convert the existing RulesLog class to a service and add a test case for it.

So:
- Let's implement https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-3-logg...
- Let's inject the core logger service, so we can forward messages above a configurable severity to the core logger
- Let's name the service rules.log