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/gcd.c

@@ -2,9 +2,12 @@ int gcd(int a, int b) {
     a = (a > 0) ? a : -a;
     b = (b > 0) ? b : -b;
 
-    while(a != b) {
-        if(a > b) { a -= b; }
-        else { b -= a; }
+    while (a != b) {
+        if (a > b) {
+            a -= b;
+        } else {
+            b -= a;
+        }
     }
 
     return a;