PWM implementation uses a timer and interrupts (FRC1), taken from
Espressif's/NodeMCU's implementation and adapted for our use.
8 channels are supported, on pins 0, 2, 4, 5, 12, 13, 14, 15.
Usage:
import machine
pwm0 = machine.PWM(machine.Pin(0))
pwm0.freq(1000)
pwm0.duty(500)
Frequency is shared (ie the same) for all channels. Frequency is
between 1 and 1000. Duty is between 0 and 1023.