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/lost_copy.c

@@ -1,9 +1,9 @@
 int main() {
-  int i = 0;
-  int result = 0;
-  do {
-    result = i;
-    i++;
-  } while (i < 4);
-  return result;
+    int i = 0;
+    int result = 0;
+    do {
+        result = i;
+        i++;
+    } while (i < 4);
+    return result;
 }