Changes the representation in Metaslab Used

Used is now set to 1, when the slab is full.  This means that the test
for
 used - 1 == 0
can be used to detect leaving full, and entering empty, reducing fast
path deallocation branchs by 1.

src/mem/sizeclass.h

@@ -8,6 +8,7 @@ namespace snmalloc
   // We use size_t as it generates better code.
   using sizeclass_t = size_t;
   //  using sizeclass_t = uint8_t;
+  using sizeclass_compress_t = uint8_t;
 
   constexpr static uint16_t get_initial_offset(sizeclass_t sc, bool is_short);
   constexpr static size_t sizeclass_to_size(sizeclass_t sizeclass);