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/test/perf/external_pointer/externalpointer.cc

@@ -33,7 +33,7 @@ namespace test
       // store object
       objects[i] = (size_t*)alloc->alloc(size);
       // Store allocators size for this object
-      *objects[i] = Alloc::alloc_size(objects[i]);
+      *objects[i] = alloc->alloc_size(objects[i]);
     }
   }