Moving forward to C++17

[emjotde]

Hi,
Does anyone have objections to moving forward and making C++17 the standard for Marian with version 1.11?

[emjotde]

If I understand correctly there is full support for C++17 with gcc 8 (Ubuntu 20.04 LTS has gcc 9 as its default), but a significant subset of C++17 seems to be available since gcc 5. Visual Studio has full support as well.

So maybe limiting to the subset that gcc 5 supports for now? Or do we rip off the bandaid?