Should the unit type `U` in Quantity[N, U] be covariant?

[erikerlandson]

trait Fruit
trait Apple extends Fruit
trait Pear extends Fruit

val q1 = 1.withUnit[Apple] + 1.withUnit[Pear]  // is q1 Quantity[Int, Fruit] ?

def f1(f: Quantity[Int, Fruit]) = { ... }

f1(1.withUnit[Apple])
f1(1.withUnit[Pear])

would likely depend on #22