support argument unpacking

[dwf]

Like hyperopt.pyll's o_len.

It's a violation of namespace separation to pass it as a kwarg to partial/PartialPlus/etc. My preferred syntax would be one (possibly involving getitem on some magic object) that keeps output_length and any other metadata parameters in their own separate box from the partial's kwargs. I think this is doable with some care.

For the sake of not cluttering the graph too much, just add an attribute on a Node. This also lets as_partialplus more easily annotate known quantities.