Rewrite digitalRead and digitalWrite in time critical code.

These functions (which all "real professionals" hate :-)) do some constant computations at each invocation. These intermediate results can be kept as local variables in calling classes. Portability must not be sacrificed.

Possible usable libraries:

* https://github.com/greiman/DigitalIO
* https://github.com/pololu/fastgpio-arduino
* https://github.com/fenichelar/Pin
* https://github.com/hippymulehead/FastDigitalPin
* https://github.com/NicksonYap/digitalWriteFast
* https://github.com/bengtmartensson/DigitalIOPins ???

Branch: `speedup`.