unconst is a small help that converts a const iterator to a non-const
iterator with the help of the container. Currently it is using the
boost iterator/range libraries.
Convert it to <ranges> as part of an effort to standardize on a single
range library. Its only user in mutation_partition is converted as well.
Due to more iteroperability problems between <range> and boost, some
calls to boost::adaptors::reversed have to be converted as well.
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