0.12.3: Compilation issues on various platforms

New features:

There was a de-seventeen-ification atempt, which was scrapped, except for:

• simple_span, which replaced the std::array used with CTAD.
• converter<char const*> and converter<char*> are now as invalid as the std::string_view version
• action_base::names_ vector is now pre-allocated for the incoming name list

Bugfixes:

• Apple Clang gets the same compilation flags as Clang, or to be exact, it does not get GCC-specific ones.
• Apple Clang libc++ does not sneak <errno.h>, it is now added to allow compilation
• MSVC 16.4.4 adds __cpp_impl_three_way_comparison, but cannot compile automatic <=> without verbose inclusion of <compare> header (while later version don't); single instance of this operator was removed for now.
• MSVC 16.4.4 does not sneak <algorithm>, it is now added, so std::max can compile.

Conan:

This is first version available on conan-center repo, as mbits-args/0.12.3.