I don't expect this to noticeably impact performance either positively or negatively for most use-cases, especially becauseIterator::collect relies on the lower bound ofsize_hint which will remain unchanged after this PR.

However the upper bound will go fromNone toSome(upper), which may benefit downstream crates that use it as a heuristic for pre-allocation size.

Note also that I forwarded the implementation ofsize_hint to the inner iterator, which means it is UAX#29 agnostic. I'm not enough of a Unicode expert to know if e.g. word boundaries can be empty, so it may not be the tightest possible upper bound for longer strings.