For tuple.map, add Tuple.Union[this.type] as upper bound for polymorphic function f

[felher]

Currently, the signature of map on tuples is:

inlinedefmap[F[_]](f: [t] => t =>F[t]):Map[this.type, F]

but it seems that it could actually be

inlinedefmap[F[_]](f: [t <:Tuple.Union[this.type]] => t =>F[t]):Map[this.type, F]

this would make it possible, for example, to map over a tuple of Ints and increment each field by one.

[SethTisue]

someone on Discord pointed out that @odersky previously suggested changing the result type of map at #15992 (comment) — perhaps that's orthogonal to this? or perhaps the two changes should be considered together?