Add some more checkers.

Change the result of `operator=` on `Mod` to return `Mod&`.  In this
example:

```c++
Mod<4, int> a;
int b = (a = 8);
```

The value of `b` will now be 0, not 8, and will equal the value of `a`.
Hopefully this is less confusing to users of this class.

src/ds/helpers.h

@@ -53,10 +53,10 @@ namespace snmalloc
       return static_cast<T>(value & (length - 1));
     }
 
-    T& operator=(const T v)
+    Mod& operator=(const T v)
     {
       value = v;
-      return value;
+      return *this;
     }
   };