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/struct2.c

@@ -1,18 +1,18 @@
 typedef struct {
-  char a;
-  struct {
-    int b[4];
-  };
-  long c;
+    char a;
+    struct {
+        int b[4];
+    };
+    long c;
 } Temp;
 
 int main() {
     const Temp temp = {1, {{2, 3, 4, 5}}, 6};
-    
+
     Temp temp2;
     temp2 = temp;
 
     int sum = temp2.a + temp2.b[2] + temp2.c;
-    
+
     return sum == 11;
 }