Have lil.asd be the top of an asdf/package-system hierarchy.

Until asdf/package-system is universally adopted (i.e. it's part of e.g. an
ASDF 3.1 release, and it's provided by Quicklisp and every implementation),
we need to :defsystem-depends-on (asdf-package-system).

Now to migrate all files in LIL, bottom up, to the new naming style...