- move from custom concept `LogOutput` to standard concept `Logging`
- remove `Log.qll` from experimental frameworks
- fold models into standard models (naively for now)
- stdlib:
- make Logger module public
- broaden definition of instance
- add `extra` keyword as possible source
- flak: add app.logger as logger instance
- django: `add django.utils.log.request_logger` as logger instance
(should we add the rest?)
- remove LogOutput from experimental concepts
