get by index

[ExpandingMan]

I not infrequently want to get itens from an OrderedDict by their index. There doesn't seem to be a public-facing API for this, I usually use dict.keys. As this is unlikely to break I suggest we add a function. Note that keys(dict)[idx] does not work because keys returns a non-indexable KeySet. It might be nice to create an indexable KeySet object, though that seems like more effort than this problem calls for.