skiplist: switch to alloc API
skiplist: switch to alloc API

Signed-off-by: Toby Lawrence <c8420cae1b43ad0b08838661745c355024c86479@nuclearfurnace.com>

crossbeam-rs/crossbeam,crossbeam-rs/crossbeam,crossbeam-rs/crossbeam,aturon/crossbeam,crossbeam-rs/crossbeam