New (streamlined) callback system

This version replaces the heavy std::function based callback system with a smaller implementation (https://github.com/luni64/staticFunctional). The change should be transparent to users, so please report any observed incompatibilities or peculiarities.

You can still configure the library to use a traditional function pointer based void(*callback)(void) callback system. But, with the new smaller staticFunctional::function based system there should be no reason to do so.

Please note: staticFunctional requires the new toolchain introduced with Teensyduino 1.58 beta. Older teensyduino versions will fall back to the function pointer based 'void(*callback)()' system automatically.