Support GCC 10+

GCC10 (not released yet - will have to take daily build or something for the time being) claims to provide support for coroutines:

https://www.phoronix.com/scan.php?page=news_item&px=Cpp20-Coroutines-GCC-10

We do want to support GCC too (in addition to CLang and MSVC). 

**IMPORTANT: it is desirable to try it BEFORE official release of GCC10.** 