share internals of R.arity and R.nAry

I don't see a reason to create some of the arity fixers up front, since each one is created at most once anyway.

I think that's me.  I'm not going to try to dig out the comment where I elucidated my rationale for it, but, while I know I'm almost certainly going to lose this battle, I still feel strongly that function declarations are a bit of an abomination that confuse the hell out of people and make them believe that functions are privileged values, somehow different from all others in the language.