Opensourcing and announcing

[atsushieno]

There is really no reason to keep this repository closed. I am yet keeping it so for couple of excuses:

• This kind of project should be strictly ABI stable and it's quite not ready for that : we keep it not announced much and loudly claim that everything is subject to change.
• Concepts are not proved workable or usable : it can be "to be proved or can result in failure".
• CI builds don't pass : they don't, but it is either GitHub Actions not supporting CMake 3.10 or bitrise Ubuntu server being too old (16.04) and custom docker image costs. If users can build it then it should be acceptable.
• The C++ code quality is awkward : but everyone writes horrible code in the beginning. I should be honest and expose anything that may result in fair evaluation on any coding skills.
• Various README parts are outdated - they are always likely.

Those excuses are mostly invalid. Yet not making any announcement makes sense.