Explicit definitions for Arduino framework setup() and loop() functions

[mairas]

I'm just linking this related ReactESP issue here for more publicity: mairas/ReactESP#16

The benefits are listed in the issue. The drawbacks that I can immediately think of would be:

• This would be a breaking change in ReactESP. Any users would need to declare the dependency to ReactESP major version to avoid breaking existing code. In platformio.ini lib_deps: mairas/ReactESP @ ^1.0.0. But at the moment nobody has that, so they'd find it out by having code that stops compiling.
• This would also be a breaking change in SensESP 2.0. Software would need to be modified to use setup() and loop() instead of the current ReactESP app(lambda); syntax.

[mairas]

Fixed in #475.