Add `values` function which is equal to `elems`

[MangoIV]

In Map k v, v stands for "value". So, since Map.keys gives you the keys of the Map, Map.values should give you the values of the Map.

I hence propose to add the following definition:

Map.values = Map.elems

[meooow25]

values might have been a better name instead of elems, but I don't think this is worth having in addition to it. We already have a confusing situation where assocs, toList and toAscList all do the same thing.

elems is the conventional name for this function today across libraries: Map and IntMap in containers, Array in array, HashMap in unordered-containers, MaxPQueue and MinQueue in pqueue. This is nice, and I wouldn't want to add values to the mix.