L0Error -> L0E
drop Level naming scheme

I was going to make distinct “levels” for abstract interpretation, but
instead I think I’ll just have “companion” functions for each concrete
function that allow information to degrade.

That is, instead of distinct “level N” implementations of addition that
either reject, accept, or mandate abstract values, there will just be
`add-concrete`, `add-with-abstract`, `add-only-abstract`.