* Enable backend functions which are *not* generators
* Use the sumfac switch as a default switch

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.

Needs a small fix in loopy (MR opened).

It is a memory reinterpretation

... instead of reimplementing it all over the place

Get endless temporaries who circularly switch around their base storage.
Any temporary may have different shape or dim_tags, reinterpreting the
data at will.