Provide a char_range method on Rope

[gchp]

Rope provides chars method which gives an iterator over all the chars in the Rope. This is great, but I find myself creating an iterator over all the data when I only really need a small subset.

Being able to get an iterator over a subset of the entire dataset would be great. I envision it something like:

// get the first 5 chars from the rope
let chars = rope.char_range(0..4);
// get chars from the 11th character to the end of the rope
let chars = rope.char_range(10..); 

I am putting together an implementation of this now, but wanted your thoughts first to make sure it's something you see value in having.

[gchp]

This is also what prompted me to submit #12 - I figured it would be best to implement it once, rather than in two places.

[gchp]

An alternative to this would be to implment iterators for RopeSlice. That would mean I could do:

let chars = rope.slice(10..20);
for ch in chars { ... }

[nrc]

Seems like a good thing to add. However, it's worth thinking about the bytes vs chars distinction - the slice approach would be bytes bases. For char_range, you'd have to iterate over the characters before the ranges in any case, so it doesn't seem a huge win over just getting the char iterator. If getting a slice of the rope by bytes is Ok for you, then that is probably a better solution.

[gchp]

That's a good point. I went with the bytes approach. Take a look at #14 and let me know what you think.