etl::message_broker + constexpr bitset

Added etl::message_broker to message framework
Added 'successor' parameter constructors to all etl::imessage_router derived classes
Added etl::binary_search to algorithm.h
Standardise conditional compilation macros for ETL_USING_CPP11 and ETL_USING_STL
#546: Added a new variant of bitset that may be constexpr
Removed the use of ETL_FORCE_CONSTEXPR_ALGORITHMS as all algorithms may be constexpr
Rationalised etl::send_message functions
Fixed implicit virtual warning for etl::vector
Fixed C++03 delegate compatibility
Added missing char_traits unit tests and char_traits bug fixes
Added default message id constructors
Added ability to set the bitset from wchar_t, char16_t and char32_t
Added ARM5 and ARM6 compiler compatibility for GCC atomics
#579: Fixed Unexpected namespace for variant with non-CPP11 compiler
#580: Fixed numeric_limits redefinition for gnu c++20