Various fixes and updates

Updates to etl::delegate. Added more constexpr and set() functions.
Removed conanfile.py. Now controlled by conan repository.
Fixed missing 'typename' keyword in etl::multi_span.
Added missing explicit initialisation from in_place structures.
Fixed hidden parameter warning in unordered_map, unordered_multimap, unordered_set, unordered_multiset.
Disable etl::begin(), etl::end() and etl::size() for STL & >= C++11.
Add available_bytes to byte_stream_reader & byte_stream_writer.
Meson tests & style fix (#447).
Fix constexpr accepts() implementation to be C++11 compliant (#451, #452)
Fix-compiler-error-msvc-16 #454

Added missing read() member functions in byte_stream_reader

Updated sanity checks for etl::multi_span

etl::multi_span, etl::byte_stream updates + others

Added front() access to locked queues.
Added .clang-format rules.
Added etl::pool_ext and etl::generic_pool_ext.
Added etl::multi_span.
Added CMSIS RTOS2 mutex variant.
Added 'unchecked' read and write members functions to byte_stream_reader and byte_stream_writer.
Fixed byte_stream_reader to be able to use const buffers.
Removed compiler warning messages.
Fixed missing 'public' access for etl::message_router_registry iterator.

Modified etl::cyclic_value for ARMCC v5 compatibility

20.17.3

Updated version numbers

Replaced std::size_t with size_t in utility.h

20.17.2

Updated versions

Added missing functionality from last commit

Added successor handling for accepts()
Added successor handling for templated receive()
Added successor handling for pre-C++17 message router

New classes + improvements.

Added etl::result type
Added etl::byte_stream
Added denominated etl::to_string overloads
Added etl::remove_cvref to type traits
Replaced etl::remove_reference_t with etl::remove_cvref_t in variadic etl::variant
Added call_if and call_or member functions to etl::delegate
Improved compliance with MISRA rules
Extended 'successor' handling for all derived message router types

Fixed incomplete template specialisations in type_traits.h for C++11

Added missing #include for variant.

Fix for etl::mutex::try_lock() for GCC

20.16.1

Fix for etl::mutex::try_lock() for GCC

C++17 variadic versions of etl::message_router and etl::fsm

Added C++17 variadic versions of etl::message_router and etl::fsm
'pop_into' in queue and stack will move if possible.
Added Force C++03 option to Github CI
Expanded compiled variants for scripts