Implement the discussed non-monotonic inheritance of propagation traits.
For backward compatibility with ASDF 2, an operation that doesn't explicitly
inherit from one of the propagation classes or non-propagating-operation
will be downward- and sideway- propagating, with a warning at instantiation.
Inheriting from non-propagating and a propagating class at the same time
yields an error, not a warning.
Update packages and tests.
Bump version to 3.1.0.56
More cleanups:
* Add some comments on TRAVERSE-ACTION; expand those on SOURCE-FILE-TYPE.
* Remove a few unnecessary input-files or output-files methods
* Remove (declare (ignorable ...)) for specialized arguments. Specialize a few arguments to T.
* Remove most when-upgrading methods, since we drop data on old incompatible ASDFs.
Bump version to 3.1.0.57