[FEATURE, EPIC] Officially Support MacOS

[ptsouchlos]

dp::thread_pool so far hasn't been tested on macOS. It would be good to add "official" support for this platform and show that it builds (reliably) on macOS. I think for now it's reasonable to only support LLVM on macOS.

Tasks

• Get dp::thread_pool building and tests passing on macOS
• Add CI build for macOS
• Add ability to alter stack size of threads #36