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/typecast.c

@@ -1,5 +1,5 @@
 char temp = 0x00L;
 
-int main(){
+int main() {
     return (temp = 0xEF36L) >= (2L);
 }