add generic function

[amir-arad]

I've added a generic function with variable amount of arguments. It's something I need in my work for function chaining, adaptors and such.

Issues:

1. type NumberToString should move to typelevel-ts project IMO
2. I couldn't find a way to get rid of the first generic param I. It looks like it's redundant, as the size of the tuple is already in the A param.
   I've tried these things:

type TupleLength<T, I = 0> = {
    'true': TupleLength<T, Increment[I]>
    'false': I
}[ObjectHasKey<T, I>]
type GFunction2<A, R=void> = GFunction<TupleLength<A>, A, number>;
type GFunction3<A, R=void, I=0> = GFunction<TupleLength<A, I>, A, number>;

and also

export type GFunction<A extends Array<any>, R=void> =
    If<ObjectHasKey<A, '1'>,
        Function2<A[0], A[1], R>,
        If<ObjectHasKey<A, '0'>,
            Function1<A[0], R>,
            Lazy<R>>>

would welcome any ideas...

[SimonMeskens]

I did some initial tests and it's perfectly possible to get rid of the first generic parameter. The code is kinda complex though, it seems. I'll report back if I find a solution I like.

[gcanti]

Interesting topic, thanks for this PR @amir-arad, though it seems more related to typelevel-ts than fp-ts, please open an issue there if you find a good solution