Use a persistent vector implementation in NamingContext

[phipsgabler]

This data structure is completely immutable anyway, but currently not space-efficient, since it cats onto vectors. Something with sharing, O(1) push, and ideally O(1) indexing is needed.

The PersistentVector from FunctionalCollections should work perfectly, but I have the impression that it is not well curated currently.

Would be interesting if there's some persistent data structure that also has better than O(n) findfirst -- something like a persistent ordered dictionary?