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/switch-in-loop.c

@@ -1,18 +1,18 @@
 int main() {
-  int i = 0;
-  int c = 0;
-  while (i < 10) {
-    i++;
-    switch (i) {
-      case (1): {
-        continue;
-        break;
-      }
-      default: {
-        break;
-      }
+    int i = 0;
+    int c = 0;
+    while (i < 10) {
+        i++;
+        switch (i) {
+            case (1): {
+                continue;
+                break;
+            }
+            default: {
+                break;
+            }
+        }
+        c++;
     }
-    c++;
-  }
-  return c;
+    return c;
 }