Remove union in metaslab

The union in Metaslab provides no benefit in size, as the single byte
it effectively saves will be removed due to padding.

By removing the union, we get stronger properties over sizeclass, and
remove an out-of-bounds access.

src/mem/metaslab.h

@@ -57,11 +57,8 @@ namespace snmalloc
     // doubly linked node into that size class's free list.
     uint16_t link;
 
-    union
-    {
     uint8_t sizeclass;
     uint8_t next;
-    };
 
     void add_use()
     {

src/mem/superslab.h

@@ -194,6 +194,7 @@ namespace snmalloc
       uint8_t index = (uint8_t)slab_to_index(slab);
       uint8_t n = head - index - 1;
 
+      meta[index].sizeclass = 0;
       meta[index].next = n;
       head = index;
       bool was_almost_full = is_almost_full();