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/array3.c

@@ -1,12 +1,12 @@
-int* foo(int a[10]){
+int* foo(int a[10]) {
     return a;
 }
 
 int main() {
     int a[10];
-    
+
     for (int i = 0; i < 10; i++) {
-	    (foo(a))[i] = i;
+        (foo(a))[i] = i;
     }
 
     return a[5] == 5;