nonwrapping_range<T> and related templates represent mathematical
intervals, and are different from C++ ranges. This causes confusion,
especially when C++ ranges and the range templates are used together.
As the first step to disentable this, introduce a new interval.hh
header with the contents of the old range.hh header, renaming as
follows:
range_bound -> interval_bound
nonwrapping_range -> nonwrapping_interval
wrapping_range -> wrapping_interval
Range -> Interval (concepts)
The range alias, which previously aliased wrapping_range, did
not get renamed - instead the interval alias now aliases
nonwrapping_interval, which is the natural interval type. I plan
to follow up making interval the template, and nonwrapping_interval
the alias (or perhaps even remove it).
To avoid churn, a new range.hh header is provided with the old names
as aliases (range, nonwrapping_range, wrapping_range, range_bound,
and Range) with the same meaning as their former selves.
Tests: unit (dev)
