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Added _stepping, _stepTime, and _lastStepTime inst vars to StackProgram. And changed constructor to use the first 4 bytes of the stream to set this variables: first bit for the _stepping, and the remaining 31 bits for the _stepTime, _lastStepTime is simply set to 0.
Changed StackVM::executeProgram(..) to check if the program is stepping and if it should step now before executing it.
Improved SerialStream::upTo(..) for the cases where the buffer size is exceeded. Now we start with a buffer of 100 bytes and we double its size if needed.
Fixed a bug in ReadStream::nextLong(..) that wasn't working when the size parameter was bigger than 2.
Changed example programs to set the new stepping and step time (and in the case of the blinkLed9 avoid calling the delay primitive).