Make Str.split(Regex) up to 2x faster
Make Str.split(Regex) up to 2x faster

- using the new R:It.MatchSplit iterator
- now *truly* lazy, so in some application potentially faster still. e.g. first
- for now, without :k, :kv, :v or :p specification
- also removed some helper private methods that are no longer needed

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