Last second fixes

Just a touch-up for the `io-page` case:

Allocator improvements can definitely wait for a future release, but I thought why not just do a cons followed by a [memmove](https://github.com/mirage/ocaml-cstruct/blob/master/lib/cstruct_stubs.c#L47), instead of a [memcpy](https://github.com/mirage/ocaml-cstruct/blob/master/lib/cstruct_stubs.c#L29), a [spin](https://github.com/mirage/io-page/blob/master/lib/io_page.ml#L74), and two conses -- right away?

Should be plenty fast. :metal:

Release away!