Cleanup static variables

1. `static const int hash_value_mask` can be changed to `static constexpr int hash_value_mask`:
https://github.com/boostorg/regex/blob/cb559132939670aa45eb25fd867fce0be8b08837/include/boost/regex/v5/basic_regex.hpp#L61

2. `static` on trivial `w` and `s` variables can be removed:
https://github.com/boostorg/regex/blob/cb559132939670aa45eb25fd867fce0be8b08837/include/boost/regex/v5/basic_regex_creator.hpp#L267-L268

3. `static const std::uintmax_t one ` can be changed to `constexpr std::uintmax_t one`:
https://github.com/boostorg/regex/blob/cb559132939670aa45eb25fd867fce0be8b08837/include/boost/regex/v5/basic_regex_creator.hpp#L1470
https://github.com/boostorg/regex/blob/cb559132939670aa45eb25fd867fce0be8b08837/include/boost/regex/v5/basic_regex_creator.hpp#L1490

4. `static const char incomplete_message[]` can be changed to `const char * const incomplete_message`:
https://github.com/boostorg/regex/blob/cb559132939670aa45eb25fd867fce0be8b08837/include/boost/regex/v5/basic_regex_parser.hpp#L852
https://github.com/boostorg/regex/blob/cb559132939670aa45eb25fd867fce0be8b08837/include/boost/regex/v5/basic_regex_parser.hpp#L1132
https://github.com/boostorg/regex/blob/cb559132939670aa45eb25fd867fce0be8b08837/include/boost/regex/v5/basic_regex_parser.hpp#L1338
https://github.com/boostorg/regex/blob/cb559132939670aa45eb25fd867fce0be8b08837/include/boost/regex/v5/basic_regex_parser.hpp#L1430

5. `static const bool value` can be changed to `static constexpr bool value`:
https://github.com/boostorg/regex/blob/cb559132939670aa45eb25fd867fce0be8b08837/include/boost/regex/v5/iterator_category.hpp#L36
https://github.com/boostorg/regex/blob/cb559132939670aa45eb25fd867fce0be8b08837/include/boost/regex/v5/iterator_category.hpp#L42
https://github.com/boostorg/regex/blob/cb559132939670aa45eb25fd867fce0be8b08837/include/boost/regex/v5/iterator_category.hpp#L75

6. `const bool is_random_access_iterator` can be changed to `constexpr bool is_random_access_iterator`:
https://github.com/boostorg/regex/blob/cb559132939670aa45eb25fd867fce0be8b08837/include/boost/regex/v5/iterator_category.hpp#L79
these two lines can be removed after change line 75 to `constexpr`.

7. `static const std::ptrdiff_t k` can be changed to `constexpr std::ptrdiff_t k`:
https://github.com/boostorg/regex/blob/cb559132939670aa45eb25fd867fce0be8b08837/include/boost/regex/v5/perl_matcher_common.hpp#L104

8. `static const bool value` can be changed to `static constexpr bool value`:
https://github.com/boostorg/regex/blob/cb559132939670aa45eb25fd867fce0be8b08837/include/boost/regex/v5/regex_traits.hpp#L59

9. big array `const char* messages[]` can be marked as `static`:
https://github.com/boostorg/regex/blob/cb559132939670aa45eb25fd867fce0be8b08837/include/boost/regex/v5/regex_traits_defaults.hpp#L53

11. big array `char_syntax[]` can be marked as `const`:
https://github.com/boostorg/regex/blob/cb559132939670aa45eb25fd867fce0be8b08837/include/boost/regex/v5/regex_traits_defaults.hpp#L158
https://github.com/boostorg/regex/blob/cb559132939670aa45eb25fd867fce0be8b08837/include/boost/regex/v5/regex_traits_defaults.hpp#L353

12. big array `combining_ranges[]` can be marked as `static`:
https://github.com/boostorg/regex/blob/cb559132939670aa45eb25fd867fce0be8b08837/include/boost/regex/v5/regex_traits_defaults.hpp#L545

13. `static const` for trivial variables can be changed to `constexpr`:
https://github.com/boostorg/regex/blob/cb559132939670aa45eb25fd867fce0be8b08837/include/boost/regex/v5/regex_traits_defaults.hpp#L897-L902

14. `static const char_class_type` can be change to `static constexpr char_class_type`:
https://github.com/boostorg/regex/blob/cb559132939670aa45eb25fd867fce0be8b08837/include/boost/regex/v5/w32_regex_traits.hpp#L411-L415

15. `static const string_type null_string;` can be changed to `constexpr string_type null_string;` `const string_type null_string;`:
https://github.com/boostorg/regex/blob/cb559132939670aa45eb25fd867fce0be8b08837/include/boost/regex/v5/w32_regex_traits.hpp#L589

Start by reviewing the referenced declarations in include/boost/regex/v5/basic_regex.hpp, basic_regex_creator.hpp, basic_regex_parser.hpp, iterator_category.hpp, perl_matcher_common.hpp, regex_traits.hpp, regex_traits_defaults.hpp, and w32_regex_traits.hpp. Apply the listed storage and constexpr cleanups, then run the Boost.Regex test suite and confirm the headers compile without behavioral changes.

Written by the indexing model from the issue text.