Types containing Links may be Send: the pointers within the Links may
mutably alias another value, but the links can only be accessed by the
owner of the List itself, because the pointers are private. As long as
List upholds its own invariants, Links should not make a type !Send.
Types containing Links may be Sync: the pointers within the Links may
mutably alias another value, but the links can only be accessed by the
owner of the List itself, because the pointers are private. As long as
List upholds its own invariants, Links should not make a type !Sync.