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/bar.c

@@ -1,8 +1,11 @@
-int bar(int x, int y, int z){
+int bar(int x, int y, int z) {
     int arith_mean = (x + y + z) / 3;
     int ugly_mean = (((x + y) / 2) * 2 + z) / 3;
-    if (x == y) { return y; }
-    else { return z; }
+    if (x == y) {
+        return y;
+    } else {
+        return z;
+    }
 }
 
 int main() {