Implement Clone, Debug for iterators

[clarfonthey]

Follow-up to #542 after I noticed these were missing too. A more complete implementation of #541.

Adds:

• explicit Clone for raw::Iter and raw::IntoIter
• derived Clone for iterators that use raw::IntoIter internally
• explicit Debug implementations for raw::Iter, raw::IterMut, and raw::IntoIter
• public iter and iter_mut methods to raw::IntoIter (only iter is needed for Debug, but I decided to add these as public since map and set IntoIter seem to have public iter methods)

[Amanieu]

This is not going to work correctly since it will end up double-freeing the underlying allocation.

[clarfonthey]

Right, that makes sense.

I'll look into actually cloning the allocation instead.

[clarfonthey]

Going to close this per: rust-lang/rust#128711 (comment)

I have some ideas on a refactor that would make implementing Clone a bit more comfortable, and I'll draft up my general plans in an issue first before implementing so folks can comment on them.

I posted some less constructive feedback earlier today, but the long story short is that I think that there need to be more safe wrappers on the internals in the code, rather than every single usage of the raw table being unsafe. I need to do more reading into the code before I make a proper writeup though, so, stay tuned.

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Also poked through the currently filed issues just to check for prior art before I start this, and #290 feels relevant here as well. Will want to link that in the issue I file. (Note to self.)