Various quality-of-life improvements (ideas from @33577 )

* Very basic `hello_main.c`
* Big starting hint to `write_c`
* Better error messages on failed test
  * TODO: also improve it for asmgen, but not sure how to do it in a good way

examples/c/simple_if.c

@@ -1,4 +1,4 @@
-int (fibonacci)(int n) {
+int fibonacci(int n) {
     if (n < 2) {
         n += 2;
     }