util: Fix iterators of BitArray and IntrusiveList

[MonsterDruide1]

Currently, a line like this causes cryptic error messages:
nn::util::BitArray::const_reverse_iterator it = arr.rbegin();

After two hours of debugging and working with C++ iterators, I found a fix: Declaring more of the elements required to build iterator_trait, and changing them to public to allow accessing them outside.

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This change is 

[ThePixelGamer]

@ThePixelGamer reviewed 2 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on MonsterDruide1).