Use an inline namespace to distinguish emitted symbols by version numbers

[franzpoeschel]

This is a trick commonly used to ensure that header-only C++ libraries which are used only internally at build-time do not emit conflicting symbols if the library is used in multiple packages, possibly in different version numbers (see e.g. the STL).

With this PR, the symbols produced by toml11 in downstream code contain the library's current version number:

> nm -gDC libopenPMD.so | grep -i toml | head -n 10
00000000005908b8 u guard variable for toml::toml11_3_8_1::color_ansi::detail::colorize_index()::index
0000000000590888 u guard variable for toml::toml11_3_8_1::detail::comment_index<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&)::index
0000000000335860 W toml::toml11_3_8_1::color_ansi::blue(std::ostream&)
0000000000335780 W toml::toml11_3_8_1::color_ansi::bold(std::ostream&)
00000000003356a0 W toml::toml11_3_8_1::color_ansi::reset(std::ostream&)
00000000003800e0 W std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > toml::toml11_3_8_1::format_key<char, std::char_traits<char>, std::allocator<char> >(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)
0000000000336fe0 W toml::toml11_3_8_1::type_error::type_error(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, toml::toml11_3_8_1::source_location const&)
0000000000336fe0 W toml::toml11_3_8_1::type_error::type_error(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, toml::toml11_3_8_1::source_location const&)
0000000000334940 W toml::toml11_3_8_1::type_error::~type_error()
0000000000334610 W toml::toml11_3_8_1::type_error::~type_error()

This avoids conflicts when trying to link two packages that both internally use toml11 in conflicting versions.
The inline namespace ensures that application programmers do not need to change any code since the effect of inline namespaces is only on the ABI, not on the API.

Note: I will not be able to respond next week. Feel free to modify the PR as needed.

TODO:

• Seems like I started this on the wrong branch, will need to reapply the changes
• Fix the tests
• examples/reflect/reflect.hpp: While this PR is generally not API-breaking, it probably does break the API for use in reflection. I could simply apply the necessary change there too, but I do not know the consequences that this would have on user code.

[ToruNiina]

toml-test is failing due to upstream changes around new features in 1.1.0. This is not an issue of your patch. Some features that caused discussion seems to be reverted from TOML v1.1.0. I will update it later. if all other tests pass, I will merge this.

[ToruNiina]

I confirmed that example/reflect works after applying your patch. now all the todos in your comment are done. merging.

thanks!

[franzpoeschel]

Thank you too!