Add Invariant[Numeric]

[kubukoz]

Here's a ready instance one can use:

implicit val invariantNumeric: Invariant[Numeric] = new Invariant[Numeric] {

  def imap[A, B](fa: Numeric[A])(f: A => B)(g: B => A): Numeric[B] = new Numeric[B] {
    def compare(x: B, y: B): Int = fa.compare(g(x), g(y))
    def plus(x: B, y: B): B = f(fa.plus(g(x), g(y)))
    def minus(x: B, y: B): B = f(fa.minus(g(x), g(y)))
    def times(x: B, y: B): B = f(fa.times(g(x), g(y)))
    def negate(x: B): B = f(fa.negate(g(x)))
    def fromInt(x: Int): B = f(fa.fromInt(x))
    def toInt(x: B): Int = fa.toInt(g(x))
    def toLong(x: B): Long = fa.toLong(g(x))
    def toFloat(x: B): Float = fa.toFloat(g(x))
    def toDouble(x: B): Double = fa.toDouble(g(x))
  }
}

Feel free to grab it if you have time before I do.

[kubukoz]

I'll do it now!

[tmccarthy]

Would there be any issues doing this for scala.math.Integral and scala.math.Fractional as well? These inherit from Numeric.