Facilities for abstracting over arity

Conversions between tuples and HList's, and between ordinary Scala functions of arbitrary arity and functions which take a single corresponding HList argument allow higher order functions to abstract over the arity of the functions and values they are passed

import syntax.std.function._
import ops.function._

def applyProduct[P <: Product, F, L <: HList, R](p: P)(f: F)(implicit gen: Generic.Aux[P, L], fp: FnToProduct.Aux[F, L => R]) =
  f.toProduct(gen.to(p))

Abstracting over arity

applyProduct(1, 2)((_: Int) + (_: Int)) should be(res0)
applyProduct(1, 2, 3)((_: Int) * (_: Int) * (_: Int)) should be(res1)