touch: add capacitive touch sensing on normal GPIO pins

Tested on the following chips/boards:

  * RP2040 (Raspberry Pi Pico)
  * ATSAMD21 (Adafruit PyBadge)
  * NRF52840 (PCA10056 developer board)
  * ESP8266 (NodeMCU)
  * ATmega328p (Arduino Uno)
  * ESP32C3 (WaveShare ESP-C3-32S-Kit)
  * FE310 (SiFive HiFive1 rev B)

The sensitivity threshold in the example may need to be adjusted per
board though, the default value of 100 typically recognizes when a cable
is being touched but the RP2040 for example is capable of doing much
more precise measurements if the power supply is sufficiently
noise-free.

examples/touch/capacitive/main.go

@@ -0,0 +1,69 @@
+// Capacitive touch sensing example.
+//
+// This capacitive touch sensor works by charging a normal GPIO pin, then slowly
+// discharging it through a 1MΩ resistor and seeing how long it takes to go from
+// high to low.
+//
+// Use as follows:
+//   - Change touchPin below as needed.
+//   - Connect this pin to some metal surface, like a piece of aluminimum foil.
+//     Make sure this surface is covered (using paper, Scotch tape, etc).
+//   - Also connect this same pin to ground through a 1MΩ resistor.
+//
+// This sensor is very sensitive to noise on the power source, so you should
+// probably try to limit it by running from a battery for example. Especially
+// phone chargers can produce a lot of noise.
+package main
+
+import (
+	"machine"
+	"time"
+
+	"tinygo.org/x/drivers/touch/capacitive"
+)
+
+const touchPin = machine.GP16 // Raspberry Pi Pico
+
+func main() {
+	time.Sleep(time.Second * 2)
+	println("start")
+
+	led := machine.LED
+	led.Configure(machine.PinConfig{Mode: machine.PinOutput})
+	led.Low()
+
+	// Configure the array of GPIO pins used for capacitive touch sensing.
+	// We're using only one pin.
+	array := capacitive.NewArray([]machine.Pin{touchPin})
+
+	// Use a dynamic threshold, meaning the GPIO pin is automatically calibrated
+	// and re-calibrated to adjust for varying environments (e.g. changing
+	// humidity).
+	array.SetDynamicThreshold(100)
+
+	wasTouching := false
+	for i := uint32(0); ; i++ {
+		// Update the GPIO pin. This must be called very often.
+		array.Update()
+		touching := array.Touching(0)
+
+		// Indicate whether the pin is touched via the LED.
+		led.Set(touching)
+
+		// Print something when the touch state changed.
+		if wasTouching != touching {
+			wasTouching = touching
+			if touching {
+				println("  touch!")
+			} else {
+				println("  release!")
+			}
+		}
+
+		// Print the current value, as a debugging aid. It's not really meant to
+		// be used directly.
+		if i%128 == 32 {
+			println("touch value:", array.RawValue(0))
+		}
+	}
+}