Callback for WiFi event is not std::function + WiFi.removeEvent has bug

[sticilface]

@igrr as, discussed.

https://github.com/esp8266/Arduino/blob/master/libraries/ESP8266WiFi/src/ESP8266WiFiGeneric.h#L38
is not an std::function.

These features are not documented.

https://github.com/esp8266/Arduino/blob/master/libraries/ESP8266WiFi/src/ESP8266WiFiGeneric.cpp#L87

Needs to be refactored, as there is no == operator for std::function, and as you mentioned better to have RAII implementation. (I did try, and you've mentioned there is an error).

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[igrr]

Created #2119 which addresses this issue.