Merge remote-tracking branch 'origin/development'
# Conflicts:
# src/cstring.h
# src/flat_map.h
# src/set.h
# src/u16string.h
# src/u32string.h
# src/wstring.h
# test/test_string_char.cpp
# test/test_string_u16.cpp
# test/test_string_u32.cpp
# test/test_string_wchar_t.cpp
# test/vs2017/etl.vcxproj.filters
