After dask/distributed#6028 we'll be able to remove a lot of the cruft around pack/unpack of high level layers. (woo!). Now is probably also a good time to rethink that interface. Currently the interface seems to be the following:

• Mapping, so getitem, list keys, len
• Cull

What would make more sense? I'd like to solicit information about what constraints on a design should be. What do we need minimally? Then after that (in a separate issue) let's think about what we might add on top.

cc @rjzamora @ian-r-rose @jcrist