Enabling std::function support for SAMD

[rueckix]

Addressing: #109

This is quite useful when using callbacks on group objects. When modelling a device as a class, the callback is typically a non-static member function that cannot be passed as a regular function pointer.
An elegant solution is to combined std::bind with std::function to register a member function of a specific class object.

Implications
The change is simple but including STL headers conflicts with the definition of max and min macros in the arduino framework for SAMD. This should be fixed in arduino/ArduinoCore-samd#399 but not released.

Per interim, bits.h is changed to do a conditional

#undef max
#undef min

[thelsing]

Merged thanks.