prefer auto to Rng::(const_)?iterator for generic String

[wanghan02]

1. c++20 ranges library does not require a range to have member type/alias iterator/const_iterator.
2. boost range library uses typename boost::range_iterator<Rng>::type instead of Rng::iterator because a customized Rng may not have member type/alias iterator/const_iterator. e.g. std::pair<It, It>.

Therefore for generic String type, auto is better than Rng::(const_)?iterator.

[wanghan02]

The changes in this commit are all in x3 folder with std14 at least. Somehow CI fails in some std03/11 karma/qi builds.