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/array.c

@@ -1,4 +1,4 @@
-int sum(int len, int *p) {
+int sum(int len, int* p) {
     int result = 0;
     for (int i = 0; i < len; i++) {
         result += p[i];
@@ -10,7 +10,7 @@ int sum(int len, int *p) {
 int main() {
     int a[5];
     int len = 5;
-    
+
     for (int i = 0; i < len; i++) {
         a[i] = i;
     }