Add 4-wire 8-step Support for 28BYJ-48 Motor

[dthain]

This PR provides support for 4-wire 8-step motors, such as the 28BYJ-48, which is very widely available now, and not well supported by this library. To enable the new 8-step pattern, construct the four-wire object as normal, then do this to enable the alternate pattern:

stepper.setBeatsPerPattern(8);

Behavior is unchanged for existing code and motors.

I confess that the terminology "beats per pattern" is a bit awkward; if anyone has a suggestion on how to more clearly represent this to the user, that would be great.

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