Enforcing assumptions

[Robert Goldman]

I have been working on enforcing assumptions recently added to the ASDF
docs.  Specifically, that OPERATION instances only be created by
MAKE-OPERATION and SYSTEMs only be parsed inside LOAD-ASD.

I should have a merge request up for review soon, but find that it's
more tricky than I expected, because we don't even play by the rules
ourselves!  Specifically, there are calls to MAKE-INSTANCE on OPERATIONs
in the ASDF codebase itself.

Cheers,
r