Rewrite CI with GitHub Actions

I see that there are both Travis and Appveyor CI files, but they seem outdated and aren't passing.

Would you be willing to review a PR that reimplemented the CI using GitHub Actions? GitHub Actions is free for public repos and supports Linux, Mac, and Windows runners.

If the above is okay, would you want to keep the CI environments exactly the same:

• compiler versions
  • Linux - GCC 5
  • macOS - XCode 9.1
  • WIndows - VS 2013, 2015, 2017
• CMake configurations (Debug and Release)
• Architectures (x86-64)
• CMake version (3.0, as specified as the minimum version in the CMakeLists.txt)

Or are there any modifications to reduce the number of jobs or update minimum versions?

Inizia esaminando i file CI esistenti di Travis e Appveyor e la versione minima di CMake in CMakeLists.txt. Prima di modificare qualsiasi cosa, chiarisci se le versioni dei compilatori, le configurazioni Debug e Release, l’architettura x86-64 e la copertura delle piattaforme debbano rimanere invariate; il lavoro sarà considerato completato quando la matrice concordata verrà eseguita correttamente tramite GitHub Actions.