machine: Rewrite most of the GPIO functionality

Split across device types (nrf, dummy) and use registers directly
instead of the HAL.

src/machine/machine_dummy.go

@@ -0,0 +1,21 @@
+
+// +build !avr,!nrf
+
+package machine
+
+// Dummy machine package, filled with no-ops.
+
+type GPIOMode uint8
+
+const (
+	GPIO_INPUT  = iota
+	GPIO_OUTPUT
+)
+
+const LED = 0
+
+func (p GPIO) Configure(config GPIOConfig) {
+}
+
+func (p GPIO) Set(value bool) {
+}