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/struct4.c

@@ -1,19 +1,16 @@
-int nonce = 1; // For random input
+int nonce = 1;  // For random input
 
-struct Foo
-{
+struct Foo {
     int x;
 };
 
-struct Foo f()
-{
+struct Foo f() {
     struct Foo x;
     x.x = nonce;
     return x;
 }
 
-int main()
-{
+int main() {
     int x = f().x;
     return x;
 }