queryset updates, prefetch improvements

[devkral]

Changes

• fix tests (inversion was never noticed)
• allow traversing nested attributes with embed_parent
• allow prefetching across unidirectional relations (and simplify it)
• prefetches are now handled in parallel
• use run_sync for prefetches instead of the loop stuff
• move crawl_relationship for preventing import loop
• remove extra
• cleanup queryset
• allow multiple clauses
• fix exclude not applied on clauses
• apply or or and on clauses too
• allow mixed clauses and kwargs

[tarsil]

@devkral Pull the latest develop, merge the main and open the PR against it, please

[devkral]

@devkral Pull the latest develop, merge the main and open the PR against it, please

I did so already. Tests will follow soon

[devkral]

I added the tests and docs. You can merge if you want or give me the ok (I will try to find time to merge).