alloc: de-static alloc_size

We're going to need to amplify the pointer and that's going to require access
to our AddressSpaceManager, which we only get non-statically through our
LargeAlloc.

This patch unto itself makes the world slower, perhaps because Clang can't see
the certainty of aliasing of the static and non-static paths to the same
structure.  However, when we also de-static external_pointer, that goes away and
things return to the status quo ante.

src/mem/alloc.h

@@ -429,13 +429,13 @@ namespace snmalloc
     }
 
   public:
-    SNMALLOC_FAST_PATH static size_t alloc_size(const void* p)
+    SNMALLOC_FAST_PATH size_t alloc_size(const void* p)
     {
 #ifdef SNMALLOC_PASS_THROUGH
       return external_alloc::malloc_usable_size(const_cast<void*>(p));
 #else
       // This must be called on an external pointer.
-      size_t chunkmap_slab_kind = ChunkMap::get(address_cast(p));
+      size_t chunkmap_slab_kind = chunkmap().get(address_cast(p));
 
       if (likely(chunkmap_slab_kind == CMSuperslab))
       {