make Bag/Mix.list more hashy, returning .pairs

Bags and Mixes are more naturally hashy, so we'll default to returning
.pairs in list context.  Set, however, still returns only .keys for its
default list, since the values are uninterestingly true.  (cf RT #121947)

S02-bits.pod

@@ -1638,8 +1638,9 @@ simultaneously, but an iterator/list itself has only one thread of
 consumption.  Every time you do C<get> on an iterator, a value
 disappears from its list.
 
-Note that C<Set>, C<Bag>, and C<Mix> iterators return only keys, not values.  You
-must explicitly use c<.pairs> to get key/value pairs.
+Note that C<Set> iterators return only the keys, not the boolean values.  You
+must explicitly use C<.pairs> to get key/value pairs.  The C<Bag> and C<Mix>
+types, on the other hand, default to returning pairs, as a C<Hash> does.
 
 =head2 Mutable types