The logger was not able to handle multiple destinations before. Now it
is possible for the logger.
The file must be held behind an Arc<Mutex<_>> so we can use the logging
from multiple threads but also because we need to borrow mutably, so
that bit changes whith this commit.
This merge solved a _LOT_ of conflicts and was a rather complicated one,
as parts of the conflict-resolution involved rewriting of half the
stuff.
This merge commit fixes all the things so a `cargo check --all`
succeeds, but I did not yet check whether tests run without failure.
This is the first part of a series of patches that reimplements the
logging backend to be more powerful and configurable.
This first patch adds types and infrastructure to be able to implement a
powerful logging abstraction.
It does not implement much functionality, except for a check whether a
module is enabled or not when logging (if configured in the config
file).
