pystencils Backend Rework

The code generation backend of pystencils is due for a major overhaul. This issue is meant to track the efforts in this direction.

Development branch: pystencils/backend-rework

The various sub-tasks of this project are documented using the Tasks below.

Documentation for this branch is currently served here: https://da15siwa.pages.i10git.cs.fau.de/dev-docs/pystencils-nbackend/