Make split_after() more generic by allowing split_point to be anywhere,
not just within the input range. If the split_point is before, the entire
range is returned; and if it is after, stdx::nullopt is returned.
"before" and "after" are not well defined for wrap-around ranges, so
but we are phasing them out and soon there will not be
wrapping_range::split_after() users.
This is a prerequisite for converting partition_range and friends to
nonwrapping_range.
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