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/swap.c

@@ -1,10 +1,10 @@
 int main() {
-  int a = 1;
-  int b = 2;
-  do {
-    int t = a;
-    a = b;
-    b = t;
-  } while (b == 1);
-  return a * 10 + b;
+    int a = 1;
+    int b = 2;
+    do {
+        int t = a;
+        a = b;
+        b = t;
+    } while (b == 1);
+    return a * 10 + b;
 }