Decouple input arrays from Slack payload format

[maknz]

At the time, having the input arrays into Attachment and friends be exactly the same as the Slack payload format was good, but now it's a hinderance when we want to be smarter about a few things, e.g. using timestamp rather than ts and a DateTime rather than an int UNIX timestamp.

It's made testing very difficult as a result, so I'll figure out a different way to do the functional tests, other than "these two arrays must match exactly" which is a huge PITA. It'll probably be inspecting each element individually, more verbose but no headaches.