Get rid of the arity-at-least struct, and replace it with just an arity

[spdegabrielle]

Get rid of the arity-at-least struct, and replace it with just an arity that abstracts over the whole arity thing -- always normalizing and includes keyword arity information. Also, make it possible to always use a procedure where an arity is expected.

from #33

[LiberalArtist]

I think the procedure-arity-mask family of functions may have reduced the need for this.