This is a patch release to fix minor issues with release 0.4.0.
- We removed a spurious 'optional' header (which was unused).
- We document std::string_view aliasing for pre-C++17 compilers.
- We made the cmake build more robust (missing git, ninja).
- When used inside another cmake project, we just build the library by default (no test or benchmark).
- We improved compatibility with 32-bit systems.