Improve efficiency of NRF52TouchSensor interrupt handler

[kevinjwalters]

The interrupt handler can be made more efficient in the NRF52TouchSensor class.

Major improvement

The innocuous looking Evt with default construction gets initialised with a timestamp using system_timer_current_time_us() which is far more heavyweight than it sounds.

From https://github.com/lancaster-university/codal-nrf52/blob/master/source/NRF52TouchSensor.cpp#L33-L39

static void touch_sense_irq(uint16_t mask)
{
    if (instance) {
        Event evt;
        instance->onSampleEvent(evt);
    }
}

This event isn't used by onSampleEvent() so could be replaced with something more efficient like.

        Event unused_evt(0, 0,0, CREATE_ONLY);
        instance->onSampleEvent(unused_evt);

I've tested that and it improves lancaster-university/codal-microbit-v2#475 (comment)

Minor improvement

Is this disable/enable of interrupts required?

codal-nrf52/source/NRF52TouchSensor.cpp

Lines 143 to 151 in 6a7a5bb

	// Reset the timer, and enable the pin input event, unless we're leaving a timeslot for a single channel to charge.
	target_disable_irq();
	timer.timer->TASKS_CLEAR = 1;
	if (channel >= 0)
	NRF_GPIOTE->CONFIG[NRF52_TOUCH_SENSOR_GPIOTE_CHANNEL] = 0x00010001 | (buttons[channel]->_pin.name << 8);
	target_enable_irq();

[kevinjwalters]

What is this for?

codal-nrf52/source/NRF52TouchSensor.cpp

Line 29 in 6a7a5bb

int lastTouchSample = 0;