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/override/malloc.cc

@@ -137,7 +137,7 @@ extern "C"
     }
 
     size = bits::max(size, alignment);
-    uint8_t sc = size_to_sizeclass(size);
+    snmalloc::sizeclass_t sc = size_to_sizeclass(size);
     if (sc >= NUM_SIZECLASSES)
     {
       // large allocs are 16M aligned.