Update build environment to Debian 10 #396
Description
Debian Buster (10) has been released July 6th, 2019.
It includes GCC-8 (unfortunately no GCC-9, which would give us GDC for free) which has full support for modern C++ features. Having it as base distribution would allow us to tests projects with modern C++ code base integrated with D, such as #394
CC @MartinNowak