Various updates & fixes

Updated CI scripts
Refactored GitHub CI workflows
Updated build script text
Added additional etl::make_lsb_mask and etl::make_msb_mask binary functions.
Mask functions are now constexpr for C++11 and above.
#779 hash.h: warnings produced with -Wfloat-equal
#781 etl::multi_span::iterator::operator *() fails if first span is empty
#780 endian enum_type produces useless-cast warnings
#783 Cannot insert existing value to full set
#786 Added u8string
#787 etl::expected doesn't compile with ETL_LOG_ERRORS