Make every available call site 'private'
e.g. typing command in telnet no longer prints to serial or WebUI
Serial input is now optional, see `TERMINAL_SERIAL_SUPPORT`.
Telnet using async server is a proper Print implementation now.
WebUI, MQTT and HTTP input is allowed to inject multiple lines at once.
Modernize our helper classes and remove legacy 2.3.0 quirks.
Replace global Terminal object and allow to separately
split, parse ARGV list and perform command search and invocation with
standalone function calls.
(but, notice that we still depend on a globally shared 'commands' list)
This greatly reduces used RAM (but, slightly increases our ROM).
Update our test suite to use namespaces and new calling convention.
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
