Rename lset#apply_monotone to lset#apply.

Also, allow #apply to take both monotone functions or morphisms, rather than
just monotone functions. The idea is that most users can just invoke apply;
whereas if the user wants better performance and understands the monotone
function vs. morphism distinction, they can use apply_morph when appropriate.

It would be possible to remove #apply_morph and have #apply choose at runtime
between acting like a morphism or a monotone function, depending on the nature
of its first argument. This would be pretty easy to implement but would require
special-casing support for #apply into the lattice-core infrastructure, which
would be a hacky layering violation. So I haven't done that for now.