Migration fold expressions and nested namespace definitions

[michaelheilmann]

As one of the last compilers, Microsoft Visual Studio Community improved its C++ 17 conformance (see https://docs.microsoft.com/en-us/cpp/visual-cpp-language-conformance). All compilers supported by Idlib now provide a very reasonable C++ 17 conformance. In particular, all compilers support fold expressions (MSVC since 15.5) and nested namespace definitions (MSVC since 15.3).

It is time for us now to have a look at Idlib and remove the formidable collection of workarounds for non-conformant compilers which are simply not necessary anymore.