Plist recursekey fix

[oh6hay]

One line description of pull request

Fixes the plist parser hanging for an exponentially long time attempting to recurse a valid PLIST that contains a very large number of unique paths to a small number of leaf nodes, using shared references

Description:

This fixes an issue with the plist parser interface's _RecurseKey method. For list elements, the _RecurseKey recurses into each list element independently, not checking if the references in a list point to a target already recursed into. This allows constructing a bplist representing a chain of lists, so that each list contains multiple references to the next element in the chain. The _RecurseKey naively recurses into each reference independently, not checking if it traverses into a node already traversed. The depth check doesn't help here as 15 is plenty of levels to construct an exponentially large amount of unique paths in the chain. This is fixed by also passing a set of already visited objects on the method call. The change also protects against cyclic references, as a treat.

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