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_for.c

@@ -1,9 +1,8 @@
-int main()
-{
+int main() {
     int i;
     int sum = 0;
-    for (i = 0; i < 11; ++i) { 
-        sum += i; 
+    for (i = 0; i < 11; ++i) {
+        sum += i;
     }
     return sum == 55;
 }