It would be nice to segment character iterators, especially for interoperability with the unicode-normalization crate. This could provide a solution to #7 when/if io::Chars stabilizes. In particular, I'd like to write a tokenizer like this:

let input: BufRead = my_input();
let tokens = input.chars().nfkc().split_word_bounds();

One issue I see is that most of the public structs provide an as_str method that returns "the underlying data (the part yet to be iterated) as a slice of the original string". This obviously won't work with streaming types.