The standard `std::latch` is very restrictive in how it can be used, and
this makes it hard to easily leverage for simple coordination between a
set of dynamically scheduled tasks, where there isn't an interesting
synchronizing "merge" or future result.
This tool makes it easy to establish a latch, hand out handles to it,
and once all are destroyed, take whatever relevant action.
Note: this is split out of a larger change that uses it. I can wait
until the use case is ready, but seemed nice to review this separately.
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Co-authored-by: Dana Jansens <danakj@orodu.net>
