In #14567 I claimed that the underlying allocation is reused. And it
was. At the time I've submitted a PR I was using `.filter_map(|x| x)`,
which got flagged by clippy as something that could be simplified to
`.flatten()` - that however broke the allocation reuse promise.
Thus, this PR goes back to using `filter_map` and additionally in debug
builds it performs checks for allocation reuse. With .flatten in place,
a bunch of unit test fail on that branch, so the checks do work.
Release Notes:
- N/A
Piotr Osiewicz
09c497f744