AI-37 DSig Core change proposal

Here's a replacement for section 5 of the core spec. I change-tracked it from Eve's last draft, but that section hasn't been touched
yet anyway.

The substance is what I cribbed from the Liberty spec and posted last week, with the basic layout adjusted to match the sections
that were there from before.

Barring edits, I'd move for a vote on this text tomorrow so we can put this to bed.

-- Scott