NFC: Introduce and switch to pal_zero

This wrapper will allow us to pass `AuthPtr<T,B> p` to zero() without needing to
write `p.unsafe_auth_ptr` to get to a `T*` inside.  Moreover, it will give us a
convenient point to assert that `B` is such that the pointer can be used to
manipulate the memory map (i.e. is not exported).

src/mem/mediumslab.h

@@ -94,7 +94,7 @@ namespace snmalloc
       self->free--;
 
       if constexpr (zero_mem == YesZero)
-        PAL::zero(p, size);
+        pal_zero<PAL>(p, size);
       else
         UNUSED(size);