While reviewing #1789, @CaseyCarter noticed several things in <chrono> that could be cleaned up. None of them are correctness issues or have ABI impact, so we're filing a followup issue.

Also while reviewing #1870, @mnatsuhara identified several good cleanups. @CaseyCarter also found some things to clean up with comments in #1870 (review).

One more issue (noticed while explaining a decltype) - the occurrences of decltype(tzdb::zones) and decltype(tzdb::links) are relics of an earlier attempt to use special allocators (which was foiled by the Standard's insistence on std::allocator here). It would probably be simpler to replace these with their actual types. Example occurrences: