Removed Simple Uses of STD Iterator

[AlexandreSinger]

See issue #2557 for more context.

Removed the uses of std::iterator that were very simple to remove. These just required declaring some types that the users of the iterators expect and removing the inheritance on the std::iterator.

The other cases in VTR which use std::iterator are a bit more complicated and may require some work to remove correctly.

[AlexandreSinger]

@vaughnbetz This has passed CI, and the warnings pertaining to these specific uses of std::iterator are gone from the GCC12 build. What do you think about these changes?

[vaughnbetz]

Thanks!