Add test program func_fibo.cpp for comparison purposes

rt/test/func_fibo.cpp

+#include <cstdio>
+
+int fibo(int n) {
+  if (n < 2) {
+    return n;
+  }
+  int a = fibo(n - 1);
+  int b = fibo(n - 2);
+  return a + b;
+}
+
+
+int main() {
+  int result = fibo(40);
+  printf("%d\n", result);
+  puts("done");
+  return 0;
+}