Made a sizecass_t to wrap the sizeclass

This is useful as codegen is nicer if we use size_t, but the semantics
is uint8_t, and is stored as that in many places in the metadata.
Ultimately should introduce a wrapper to check this invariant.

src/test/func/sizeclass/sizeclass.cc

@@ -2,7 +2,7 @@
 #include <snmalloc.h>
 
 NOINLINE
-uint8_t size_to_sizeclass(size_t size)
+snmalloc::sizeclass_t size_to_sizeclass(size_t size)
 {
   return snmalloc::size_to_sizeclass(size);
 }
@@ -17,7 +17,7 @@ int main(int, char**)
 
   std::cout << "sizeclass |-> [size_low, size_high] " << std::endl;
 
-  for (uint8_t sz = 0; sz < snmalloc::NUM_SIZECLASSES; sz++)
+  for (snmalloc::sizeclass_t sz = 0; sz < snmalloc::NUM_SIZECLASSES; sz++)
   {
     // Separate printing for small and medium sizeclasses
     if (sz == snmalloc::NUM_SMALL_CLASSES)