Fixes for etl::span, etl::expected and etl::variant

• #737 etl::equal specialisation for etl::span is deduced wrongly when using STL and CPP20. Added C++20 four iterator std::equal variants.
• #738 etl::expected assignment operator won't work when TValue and TError are the same type. Changed assignment from etl::unexpected to use variant::emplace instead of direct assignment.
• #739 Could not compile ETL with gcc 9.4.0 (C++11). Added compatibility for GCC-9 + C++11. Added missing return reference from emplace member function in variant_variadic.h
• Removed many -02 warnings for GCC-13