Build a cache internal to pull spec evaluation in order to deduplicate subframe evaluation

[tylerwilding-wk]

Internally, during a pull evaluation, generated subframes may be duplicated an arbitrary number of times. Consider the spec [{:ref [*]}] being evaluated on 10000 distinct entities which all have a :ref to the same single entity. There will be 10000 distinct subframes generated for that entity, 9999 of which could be eliminated and the single output simply repeated in the construction of the pull's final output.

An internal cache of [eid spec], (or perhaps even something more granular, like [eid attr]) which could greatly reduce the total amount of IO required of pull in many cases.