A contention-free STM hash map. "Contention-free" means that the map will never
cause spurious conflicts. A transaction operating on the map will only ever
have to retry if another transaction is operating on the same key at the same
This is an implementation of the /transactional trie/, which is basically a
/lock-free concurrent hash trie/ lifted into STM. For a detailed discussion,
including an evaluation of its performance, see Chapter 4 of
<https://github.com/mcschroeder/thesis my master's thesis>.