Update Nat test to limit scope of the auxiliary N <: Nat.
Update Nat test to limit scope of the auxiliary N <: Nat.

isaka/shapeless,milessabin/shapeless,wheaties/shapeless,clhodapp/shapeless,fthomas/shapeless,triggerNZ/shapeless,liff/shapeless,lambdista/shapeless,lukasz-golebiewski/shapeless,alexarchambault/shapeless,TomasMikula/shapeless,khronnuz/shapeless