Add rudimentary interface support

src/examples/hello/hello.go

@@ -22,12 +22,25 @@ func main() {
 
 	thing := Thing{"foo"}
 	println("thing:", thing.String())
+	printItf(5)
+	printItf(byte('x'))
 }
 
 func strlen(s string) int {
 	return len(s)
 }
 
+func printItf(val interface{}) {
+	switch val := val.(type) {
+	case int:
+		println("is int:", val)
+	case byte:
+		println("is byte:", val)
+	default:
+		println("is ?")
+	}
+}
+
 func calculateAnswer() int {
 	seven := 7
 	return SIX * seven

src/runtime/runtime.c

@@ -26,6 +26,10 @@ void __go_printint(intgo_t n) {
 	putchar((n % 10) + '0');
 }
 
+void __go_printbyte(uint8_t c) {
+	putchar(c);
+}
+
 void __go_printspace() {
 	putchar(' ');
 }