Consider alternative to one-way become for PP2UnresolvedNode

[jgfoster]

GemStone doesn't support one-way become so this isn't portable. I've been able to get what appears to be equivalent functionality from a proxy/delegate pattern (though it doesn't pass the #'==' tests).

[kursjan]

This is by no means important part of PP2. I think most of the parsers ever created can hapily live without this node or calling this method.

Long term, I will try to refactoring this into different package, or, as suggested, try to use proxy pattern if doable...