SI-9382 Privatize enhanced x in Tuple2Zipped.Ops

[som-snytt]

Probably there should be an Abide rule to avoid leaking
the "underlying value" of a value class. The spec or SIP defines
"underlying type" but doesn't mention the underlying value.

The argument for concealing the member is that it is redundant
and makes autocompletion results harder to read. Also, possibly
an additional implicit might want to add a member so-named.

[adriaanm]

LGTM

[som-snytt]

@adriaanm Maybe you could sponsor me for a merit badge.

I noticed (xs, ys).zipped.unzip is non-trivial, so (xs, ys).zipped.colls is useful, even if the name is a bit casual.

Edit: alternatively, you could just get really annoyed.

[lrytz]

the name is a bit casual

so is the type

scala> (List(1,2,3), List(4,5,6)).zipped.colls
res2: (scala.collection.TraversableLike[Int,List[Int]], scala.collection.IterableLike[Int,List[Int]]) = (List(1, 2, 3),List(4, 5, 6))