Optional Tests

This release retains the sharp edges of RC1 but it works for us as a polyfill.

v22.07.0-rc1

Tidied up the repo, implemented CI and packaged headers for release.

Note: this works for us as a Clang polyfill but this is not a polished release and there are sharp edges.

Full Changelog: https://github.com/StirlingLabs/jthread/commits/v22.07.0-rc1