Switch from str::Chars to io::Chars

[IvanUkhov]

When io::Chars lands on stable, consider switching Reader from str::Chars to io::Chars so that the crate could truly work with abstract Readers and Writers instead of in-memory Strings.

[cole-miller]

Based on the discussion in that issue it looks like Read::chars is now deprecated and io::Chars will not be coming to std after all (unless I'm misreading something).

[IvanUkhov]

Yeah, I was checking it recently and came to a similar conclusion. Then there must be some other preferred way of doing this.