restructure results module

Follow up on: https://mattermost.mpi-sws.org/rts/pl/hh73rhiad7bp7pgni1oqis5mca

we need to rethink how to organize the prosa.results module. We're accumulating quite a number of top-level results,

Proposal: introduce a module prosa.results.rta to collect all response-time analyses. Below that, we can have prosa.results.rta.<processor model>.<policy>.<preemption-model> to collect the individual RTAs.