Introduce Itertools::owned() / Owned.

[hadronized]

This is very similar to std’s Iterator::clone() / Cloned, but works on more types, such as CStr / CString.

[phimuemue]

Is this essentially iterator.map(ToOwned.to_owned)?

If so:

• We are reluctant to include even more "special-case map instances" (relevant discussion e.g. Add map_err and err_into iterator adapters #714 (comment)). I support these arguments and would prefer to not pull it into Itertools.
• If someone else wants to merge, it should probably be implemented in terms of MapSpecialCase.

[hadronized]

Understood, I’ll wait for others to comment to decide what to do but I’m fine to close if you think it’s not a good fit.