[boost] Assuming maintainership of boostorg/io

I'd like to take on maintainership of IO, as suggested by Peter. So far, I've already: * Implemented the Fundamentals TS v2 ostream_joiner as boost::io::ostream_joiner * Converted the documentation to Asciidoc * Updated the test cases to use Lightweight Test and build faster * Updated the CI configurations I also plan to: * Finish the boost::io::quoted implementation started by Beman but currently in detail. i.e. Update and improve the implementation. Add better documentation and more tests, and make it non-detail (public). * Migrate my boost::ostream_string facility from Boost.Utility (which is more general purpose) to Boost.IO (because, well, IO). * Make some minor improvements to the implementation of boost::io::ios_state. Glen