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

tokuhirom/specs,tokuhirom/specs,tokuhirom/specs,prammer/specs,perl6/specs,prammer/specs,prammer/specs,perl6/specs