Prepare for release with Boost 1.80

From: Mateusz Loskot (mateusz_at_[hidden])
Date: 2022-05-12 07:59:59

Hi,

After discussion about incorporating the long backlog of changes
from develop to master for release with Boost 1.80,
I've started the preparation here
https://github.com/boostorg/gil/issues/667

I have completed the grand true merge in one shot,
so the current master should be equivalent of the develop.

I have NOT collected the release notes yet.

Your help will be greatly appreciated,
...building and testing the master branch,
submitting documentation fixes and updates
(against develop) branch, etc.

Best regards,
-- 
Mateusz Loskot, http://mateusz.loskot.net