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.

.clang-tidy

@@ -1 +1,7 @@
-Checks:          '-*,clang-diagnostic-*,google-readability-casting,readability-else-after-return,performance-unnecessary-copy-initialization,bugprone-use-after-move,modernize-use-nullptr'
+Checks:          '-*,clang-diagnostic-*,google-readability-casting,readability-else-after-return,performance-unnecessary-copy-initialization,bugprone-use-after-move,modernize-use-nullptr,modernize-redundant-void-arg,modernize-return-braced-init-list,modernize-use-default-member-init,modernize-use-equals-default,modernize-use-equals-delete,modernize-use-nodiscard,modernize-use-override,cppcoreguidelines-avoid-goto,misc-unconventional-assign-operator'
+# It would be nice to enable:
+# - readability-magic-numbers
+# - modernize-avoid-c-arrays
+CheckOptions:
+  - key:             modernize-use-default-member-init.UseAssignment
+    value:           '1'

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;
     }
   };