Logging library

The logging library is in reasonable shape, but has a long to-do list. In addition, it could use

• Performance improvements. See Log4J 2.0 for information on really fast logging.
• Async logging using an asynchronous queue from the concurrency library.
• A non-textual back-end. This shouldn't be too difficult to add since it was considered during design, but it hasn't been tested so some changes to the core logging code may be necessary.
• Documentation improvements (https://github.com/dylan-lang/opendylan/issues/576)