Major refactor, better tests, and a more efficient method for named components

• Refactored the entire thing
• Moved to a more efficient method for component names, using anonymous unions instead of references
• Removed option to disable named components as they no longer incur a performance penalty
• Added more extensive (though yet not extensive enough) tests
• Made the contents of the dd::types namespace available in dd, enabling you to write code such as dd::int2d while still allowing using namespace dd::types without exposing unnecessary symbols
• Added proper documentation
• Added automatic documentation building on push
• Moved header-file to its own include-directory, as is praxis
• Renamed the internal namespace from detail to impl
• Moved to using CMake for building tests
• Added helper-metafunctions in the dd::traits namespace, such as type_identity (function returning type argument unchanged) and require (alias for std::enable_if_t with second parameter defaulted to bool). Neither of these will be needed for C++20 or later