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