improve outstanding for lists which mismatch lengths

[matt-beanland]

When developing ash_outstanding livebook I found that outstanding could be improved for lists (in this case of structs) which mismatch length.

Currently outstanding is simply the expected list in this scenario, which is mostly correct, but not always the case.
It will be quite common for structs to have ignored fields which should not be present in outstanding even if they are present in expected.

outstanding should be calculated on each expected list item, using actual list items truncating/padding with nil as necessary, with outstanding assembled into a list.

[matt-beanland]

Added/changed some test cases in list_test.exs and list_of_map_test.exs.
Rewrote list.ex defoutstanding, and improved Outstand.suppress so that lists of all nils is treated like nil. This is used if nothing outstanding on lists of equal length. Ended up being a bit more complex that I expected.

A list containing only nils can be returned by Outstanding on list, when outstanding is nil for all items in expected, but there are extra items in actual.

[matt-beanland]

Updated the livebook and readme.

[matt-beanland]

resolved by #19