No Arduino.h

[mcspr]

Tracking branch removing Arduino.h dependency

• String -> std::string
• String(x) -> sprintf(buffer, x)
• fix union ctors & dtors
• update callbacks to use cstrings (since we still want arduino to be able to read those). no string_view's yet :(

[mcspr]

fwiw it is the same binary size for the current dev of espurna, RAM is slightly better. main reason for that is terminal implementation changes, where unordered_map is no longer included. turns out, string is about the same size

only significant issue is rpn_value:toString() for floats, since floats formatting is completely disabled there (but, it simply does String(double value, 10) instead of calling the build-in method). not really sure how to pull this off without the correct printf, besides pulling dtostrf code right here.

note that std::to_string will use printf as well, if it did actually work for the platform
(to be specific, it uses vsnprintf + alloca, which is.... interesting choice, wonder why it is that way)