The backend selecting mechanism needs all modules to be imported.
This commit restructures the package to be able to do that. Placement
of some functions felt weird, but this can still be changed.
No more placements in __init__.py anymore though, as this will
result in cyclic dependencies.

Very wild, totally WIP.

and an example with stokes stress

including CallInstructions for accumulation etc.

targetting better indexing logic.

reductions may contain inames in a more complex fashion, here.