runtime: Implement GPIO output

Now we can actually blink a LED!

src/machine/machine.go

@@ -0,0 +1,26 @@
+
+package machine
+
+// #include "../runtime/runtime_nrf.h"
+import "C"
+
+type GPIO struct {
+	Pin uint32
+}
+
+type GPIOConfig struct {
+	Mode uint8
+}
+
+const (
+	GPIO_INPUT  = iota
+	GPIO_OUTPUT
+)
+
+func (p GPIO) Configure(config GPIOConfig) {
+	C.gpio_cfg(C.uint(p.Pin), C.gpio_mode_t(config.Mode))
+}
+
+func (p GPIO) Set(value bool) {
+	C.gpio_set(C.uint(p.Pin), C.bool(value))
+}