Should `std::str::CharRange` be public?

[Kimundi]

Currenty, str exports both CharRange and methods for getting an CharRange at an specific byte index of the string.

However, with Iterators we might not really need those to be public, and if we get an encoding module they might have to migrate to there anyway, as they are specific to the utf8 representation of a string.

[bluss]

Encoding support doesn't change that str is using UTF-8 for its internal representation, and that this is also exposed in some parts of its interface, including when slicing (using byte indices). I think .char_range_at has a role for manual traversal through the string, even though it's not supposed to be used often.

[aturon]

Nominating for 1.0-beta P-backcompat-libs: we need to finalize the story around these methods.

[pnkfelix]

1.0 beta, P-backcompat-libs, I-needs-decision.

[alexcrichton]

This is now explicitly behind its own feature gate so I'm going to de-milestone this issue as we will probably not stabilize it for 1.0 at this rate.

[huonw]

Closing in favour of the tracking issue for the feature gate: #27754.