std++ 1.6.0 release notes

Creating a new std++ release will fix #119 (closed), and allow us to release Iris 3.5.0.

This is a pretty big release, with over 400 commits from 14 authors!

I got the author list with git shortlog --summary coq-stdpp-1.5.0..HEAD | gsed -E 's/[ \\t]+/ /g' | cut -d' ' -f3-100.