Add a `for_` operator (#887)

Following Elvis's original branch, I added it to a `gel.qb` module.
I also, on Elvis's suggestion, modified `std` to re-export everything
from `gel.qb`.

I renamed his `foreach` to `for_` to better match EdgeQL.

Fixes #729.

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Co-authored-by: Elvis Pranskevichus <elvis@geldata.com>