first batch of sigs

We have very few optional arguments in Ramda, but the `arity` argument to `curry` is one of them.  Should we annotate that somehow in the sig?

        * @sig (* -> a) -> Number? -> (* -> a)

0r with two different `@sig` lines:

        * @sig (* -> a) -> Number -> (* -> a)
        * @sig (* -> a) -> (* -> a)

Of course `curry` is one of the few places also where the `sig` really tells us nothing useful, so perhaps it's not worth worrying about.