Making str allocate less #3: split_*

[Kimundi]

• Most functions that used to return ~[~str] for a list of substrings got turned into iterators over &str slices
• Some cleanup of apis, docs and code layout

[Kimundi]

Hat tip to @dbaupp, who also started some of that work

[huonw]

@Kimundi, thanks, but the work is entirely yours :)

Just looking at the code, it seems like it would worth leaving the methods that return ~[~str], since the for string.each |s| { list.push(s.to_owned()) } pattern comes up a lot.

[Kimundi]

@dbaupp Yeah, I might re-introduce the old names once strcat's Iterators are in core. Ideally you could then chain those with map() etc like you can with vectors, but for now it's probably a good idea to at least guide new code to not allocate as much. :)