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/pointer.c

@@ -1,15 +1,15 @@
-int* foo(int *a){
-   return a;
+int* foo(int* a) {
+    return a;
 }
 
-int main(){
+int main() {
     int a = 1;
-    int *p = &a;
-    int **p2 = &*&p;
-    int *p3 = *&p;
+    int* p = &a;
+    int** p2 = &*&p;
+    int* p3 = *&p;
 
     *&*foo(*p2) += 1;
     *foo(p3) += 1;
-    
+
     return a == 3;
 }