mediumslab: stop assuming page-alignedness of objects

With large pages (e.g. the 64K that Debian defaults to for ppc64), this
is a bit much to ask.  It's only not true for the bottom few medium size
classes, tho', as all sizes above 256K are multiples of 64K with the
current two mantissa bits size schedule.

src/mem/alloc.h

@@ -733,12 +733,6 @@ namespace snmalloc
         size_t size1 = sizeclass_to_size(sc1);
         size_t size2 = sizeclass_to_size(sc2);
 
-        // All medium size classes are page aligned.
-        if (i > NUM_SMALL_CLASSES)
-        {
-          SNMALLOC_ASSERT(is_aligned_block<OS_PAGE_SIZE>(nullptr, size1));
-        }
-
         SNMALLOC_ASSERT(sc1 == i);
         SNMALLOC_ASSERT(sc1 == sc2);
         SNMALLOC_ASSERT(size1 == size);

src/mem/mediumslab.h

@@ -85,11 +85,10 @@ namespace snmalloc
       void* p = pointer_offset(this, (static_cast<size_t>(index) << 8));
       free--;
 
-      SNMALLOC_ASSERT(is_aligned_block<OS_PAGE_SIZE>(p, OS_PAGE_SIZE));
-      size = bits::align_up(size, OS_PAGE_SIZE);
-
       if constexpr (zero_mem == YesZero)
-        memory_provider.template zero<true>(p, size);
+        memory_provider.zero(p, size);
+      else
+        UNUSED(size);
 
       return p;
     }