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/for_continue_break.c

@@ -1,9 +1,10 @@
 int foo() {
     int sum = 0;
 
-    for(int i = 0; ;) {
-        if(i == 5) break;
-        if(i == 3) {
+    for (int i = 0;;) {
+        if (i == 5)
+            break;
+        if (i == 3) {
             i++;
             continue;
         }