par_iter for Range*<char>

[CAD97]

With rust-lang/rust#72413, ranges of char are now iterable (in 1.45, releasing 2020-07-16). Rayon should support iterating them in parallel just like normal continuous integer ranges.

I've done experimentation previously, which suggested that chain(chars before surrogate range, chars after surrogate range) is the optimal ParallelIterator for Range*<char>.

I've very little experience with implementing parallel iterators, however, so that could be untrue.

[cuviper]

Sure, we can add that.

A chain sounds fine, but I wouldn't necessarily encode that in the iterator fields. It could just use rayon::range::Iter<char>, and then in its specific fn drive it can forward to something like (start as u32..end as u32).into_par_iter().map(/*to char*/).drive(..), or chained if needed to skip surrogates. Point is, that implement choice can be dynamic.