* 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>`