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/simple.c

@@ -1,7 +1,6 @@
-int nonce = 1; // For random input
+int nonce = 1;  // For random input
 
-int main()
-{
+int main() {
     int x = nonce;
     return x;
 }