formatter: fix default list/dict generator to be evaluated more than once
Before, _hybrid.gen must be a generator which could be consumed only once.
It was okay in templatekw.py since template keywords are functions which
create temporary hybrid objects, but the formatter doesn't work in that way.
To work around the issue, this patch makes _hybrid.gen optionally be a
function returning a generator.
Thanks to Pulkit for finding this issue.
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