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/array5.c

@@ -5,10 +5,10 @@ int main() {
     int a[5] = {init, 2, 3, 4, -5};
     int sum = 0;
 
-    for(int i = 0; i < 5; i++) {
+    for (int i = 0; i < 5; i++) {
         sum += a[i];
         sum += g_a[i];
     }
-    
+
     return sum;
 }