We will use the thread safe version for implementing persistent hash maps.
The hash maps will be used to implement decision procedures and refactor
the congruence closure and ematching modules.
The persistent hash maps based on thread safe parrays are performant
when most of the time there is a single thread updating them.
We use a small hack to make sure we don't have any overhead for
parray<T, false>
i.e., the thread unsafe version used in the VM.
