Guide: iterators

An introduction to iterators. I kinda like this, but I kinda don't. Hmmm.
You don't have to change this, but I'll just point out there's two possible meanings of copy in this context:

- a byte-copy, which happens if the data was moved out of the slice
- a semantic copy (e.g. calling `clone`), which happens if the data isn't moved out of the slice

(NB. the latter means we don't actually have to be its owner.)