localalloc::dealloc tweak size math for large objects

Avoid computing bits::next_pow2_bits(1 << n).  Even if the compiler can see
through enough of the algebra, it's surely more direct to just use n.

While here, slightly expand documentation about what's going on with the
"sizeclass" encoded into MetaEntry-s.

src/mem/sizeclasstable.h

@@ -165,6 +165,16 @@ namespace snmalloc
     return bits::one_at_bit(MIN_CHUNK_BITS + sizeclass);
   }
 
+  /**
+   * For large allocations, the metaentry stores the raw log_2 of the size,
+   * which must be shifted into the index space of slab_sizeclass-es.
+   */
+  inline static size_t
+  metaentry_chunk_sizeclass_to_slab_sizeclass(sizeclass_t sizeclass)
+  {
+    return sizeclass - MIN_CHUNK_BITS;
+  }
+
   inline constexpr static uint16_t
   sizeclass_to_slab_object_count(sizeclass_t sizeclass)
   {