Generalize Mapped/Comapped/NatTRel to arbitrary products.

[milessabin]

[OlivierBlanvillain]

Hi Miles! What do you mean by "arbitrary products"? We tried to use Mapped/Comapped for a trait Foo[A, B] with one of the type parameter fixed, but the compiler does not seems to like it. @kanterov did a Comapped2 to work around the issue (see discussion here https://github.com/adelbertc/frameless/pull/18/files#r55936327), I was wondering if this is what this issue is about.

[joroKr21]

Shapeless 3 is here: https://github.com/typelevel/shapeless-3