rpc_test: Read/write null terminator as part of string
Replace Serial.print with printf in some examples

rpc_test, readback_test, mem_test
sprites: Test collision detection
sprites: Move use of printf until after initialization

Now that it's initialized by the DuinoCube library, any calls to printf()
should follow the call to DC.begin().
sprites: Improve logging of cycle time

Report it infrequently to reduce the amount of logging.
Also, properly initialize the start time to the current time.