Use q20::is_sorted to statically assert that static arrays are sorted

Several of our static arrays are supposed to be sorted (because, e.g., we're running lower_bound() or binary_search() on them). At least the entries array has, in the past, violated sorted-ness (due to a null entry at the end).

Traditionally, this sort of thing would have been checked by an assert at runtime, but C++20 gave us constexpr algorithms, so we can now static_assert(is_sorted(~~~)), too, so we should use it to catch such mistakes earl(y|ier).

Acceptance criteria:

• q20::is_sorted, a backport of C++20's is_sorted is merged
  • just the classic version, not the std::ranges one
• any assert(is_sorted) on static const arrays have been changed to static_assert(q20::is_sorted))
• all static const arrays which are supposed to be sorted got a static is_sorted check added
  • indications that an array is supposed to be sorted include:
    1. a code comment to that effect
    2. std::lower_bound, std::upper_bound, std::equal_range being called on it
    3. std::binary_search being called on it
    4. a hand-written binary search is used on the data
       • try to rewrite these to use lower_bound() instead, implementing binary search correctly is surprisingly hard