rename program_logic.{ownership -> wsat}. It really is about world satisfaction and invariants more than about ownership.

(forgot to add this to the previous commit...)

Initial proof by Jan-Oliver Kaiser, adapted by Robbert Krebbers.

Initial proof by Jan-Oliver Kaiser, adapted by Robbert Krebbers.

docs: move more things to their right place: timeless, persistent; also define and give rules for ghost ownership, view shift, WP