* Implements a selection of support machinery in `std::ranges` and adds the `<ranges>` header. Primarily consists of the range access customization point objects:
* `ranges::begin`
* `ranges::end`
* `ranges::cbegin`
* `ranges::cend`
* `ranges::rbegin`
* `ranges::rend`
* `ranges::crbegin`
* `ranges::crend`
* `ranges::size`
* `ranges::empty`
* `ranges::data`
* `ranges::cdata`
and range concepts:
* `ranges::range`
* `ranges::output_range`
* `ranges::input_range`
* `ranges::forward_range`
* `ranges::bidirectional_range`
* `ranges::random_access_range`
* `ranges::contiguous_range`
* `ranges::sized_range`
* `ranges::view`
* `ranges::common_range`
and the associated type aliases:
* `ranges::iterator_t`
* `ranges::sentinel_t`
* `ranges::range_value_t`
* `ranges::range_reference_t`
* `ranges::range_difference_t`
* `ranges::range_rvalue_reference_t`
* Adds `<ranges>` - which is mostly empty since the support machinery is defined in `<xutility>` so as to be visible to `<algorithm>`
* Annotates [P0896R4](https://wg21.link/p0896r4) as partially implemented in the "`_HAS_CXX20` directly controls" section of `<yvals_core.h>`.
* Touches `<regex>`, `<set>`, and `<unordered_set>` to add partial specializations of `ranges::enable_view` for `match_results` and `(unordered_)?multi?set` as mandated by the WD to override the heuristic.
* Partially implements [P1474R1 "Helpful pointers for `ContiguousIterator`"](https://wg21.link/p1474r1):
* Push `pointer_traits` from `<xmemory>` and `to_address` from `<memory>` up into `<xutility>`
* Add `to_address` expression requirement to `contiguous_iterator` concept, and update `P0896R4_ranges_iterator_machinery` appropriately
* Implement the changes to `ranges::data` (but not `view_interface` since it isn't yet implemented)
* Drive-by:
* Simplify the definition of `pointer_traits::_Reftype` by eschewing `add_lvalue_reference_t`.
* Strengthen `reverse_iterator`'s constructors and `make_reverse_iterator` so `ranges::rbegin` and `ranges::rend` can be `noexcept` in more cases
* Since we're using `_Rng` as the template parameter name for models of `std::ranges::range`, rename a local variable `_Rng` to `_Generator` in `<random>`
Casey Carter
Stephan T. Lavavej