* Make the library modular usable.
* Switch to library requirements instead of source. As source puts extra source in install targets.
* Add requires-b2 check to top-level build file.
* Bump B2 require to 5.2
* Change all <source> references to <library>.
* Update copyright dates.
* Move inter-lib dependencies to a project variable and into the build targets.
* Switch to /boost/test//included target for header only mode of Boost.Test.
* Update build deps.
* Add GHA CI with simpler working one based on alandefreitas/cpp-actions utilities.
* Move project global include to target local include.
* Only msvc gets minor version tests.
* Remove GHA CI to undo move to cpp-actions.
