Rationale: reserve the behavior folder for trace-based semantics.

These lemmas really constitute an analysis of the basic consequences
arising from the chosen semantics and hence logically belong to the
"analysis" part of Prosa.

To allow reasoning about an entire class of types of schedules /
processor modules, it's useful to have named definitions for various
invariants that processor models ensure. Let's collect these centrally
where we introduce processor models and schedules.

simplify structure of behavior, move arrival_sequence and schedule to toplevel, move task and sequential to model