New features, updates and fixes

#663 Make unit tests compile for C++20
#675 Compilation error in optional.h
#684 Added sanitization fix for MinGW
#687 Build failure. constexpr keyword before template keyword
#689 Add as_bytes and as_writable_bytes
#690 Add rvalue accessors to expected and optional and update constructors
#692 operator [] for flat_map always calls default constructor for mapped_type
#695 Visiting a const variant (variadic)
#698 Behaviour of bidirectional_link nodes. Links are now cleared when unlinked
#699 Optional(POD) operator bool explicit
Optimised map/flat_map/unordered_map [] operator and insert()
Removed [] operator for reference_flat_map
Unit tests compile for C++11
Fixed C++11 constexpr compatibility
Removed redundant 'const' in template parameters
Updated CI scripts to enable C++ standard and optimisation selection
Updated static constant definitions
Fixed incorrect return types for reverse iterator begin() & end()
Updated C++ standard conditional compilation for reverse iterator functions
Fixed latest Address Sanitizer issues
Message packet unused parameter fix
Added ETL_OR_STD17 macro as the namespace for size()
Added Tasking compiler
Fix type traits generator
Fix test array sizes
Changed some ETL_ASSERT macros to ETL_ASSERT_OR_RETURN
Updated CMake files to allow C++ standard selection
Replaced ETL_ASSERT_AND_RETURN with ETL_ASSERT_OR_RETURN
Changed native char8_t check
Renamed Github vs2022.yml to visual-studio.yml
Updated Unittest++ checks
Comments and noexcept updates to placement new
etl::variant (variadic) refactor and updates for C++ standards compatibility
etl::overload available in C++17 only
map/flat_map speed optimisations
Various C++03 compatibility updates for maps
Refactored Bash compilation scripts to allow selection of C++ standard and optimisation level
Renamed Bash compilation scripts. Changed 'sanity checks' to 'syntax checks
Some internal constant case changes from all upper to capital case