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/array4.c

@@ -1,9 +1,9 @@
 int main() {
     int a[10];
-    int *p = a; 
-    
+    int* p = a;
+
     for (int i = 0; i < 10; i++) {
-	    *(p++) = i;
+        *(p++) = i;
     }
 
     return a[5] == 5;