Resolve the issue with the UnixHostDuino not really being compatible
with the esp8266 Core String (...and the rest of the Core, as well)
Port the CMakeLists.txt from the rpnlib and update it use FetchContent
instead of either manually fetching dependencies or using PIO artifacts
Caching is *expected* to work, but might need slight adjustments
- remove .travis.yml
- ./travis_{script,install}.sh -> ./ci_{script,install}.sh
- flush stdout in test script to see logs more promptly
- adjust git config to ignore 'detachedHead' warnings