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This change allows specification of the idle level and TX carrier output
level (through changed initialisation API), and more flexible specification
of pulses for write_pulses.

This is a breaking change for the esp32.RMT constructor API.  Previous code
of this form:

    esp32.RMT(..., carrier_duty_percent=D, carrier_freq=F)

will now raise an exception and should be changed to:

    esp32.RMT(..., tx_carrier=(F, D, 1))