Improved CI and testing framework

[foonathan]

Extend testing framework to handle compilers without most of the support, by:

• specifying one source file per feature, instead of main including everything
• choosing the highest supported flag instead of relying on C++17
• selecting only the feature checks that do not rely on unsupported features (like C++11 features for C++14 ones)

Improve automatic CI:

• more gcc versions on Travis
• clang
• MSVC