P0355R7 Extending <chrono> to Calendars and Time Zones

Notable changes:

 * Reordered synopsis to match detailed description.
 * Significant changes to stable names, per style guide.
 * Added missing zoned_traits to <chrono> synopsis.
 * Removed redundant chrono:: qualification in synopsis.
 * [time.duration.io] Significant wording changes for comprehensibility.
 * Adjusted return type of clock_cast in synopsis to match that in
   detailed description.
 * Added subheadings to clock_cast description, replacing line comments
   in the middle of the wording.
 * Adjusted specification of clock_cast to avoid awkward "at least one
   of [...] exactly one of" construction and to avoid references to
   bullet numbers in the body text.
 * Reordered parsing and formatting sections to the end.
 * Removed descriptions for operator!=, operator>, operator<=,
   operator>= that are covered by [operators]
 * Removed some tutorial front-matter and design discussion from "day",
   "month", "year", "weekday", etc.
 * operator-(month, month), operator-(weekday, weekday): specify
   returned value in Returns: element rather than splitting the spec
   across Returns: and Remarks:.
 * Added subheadings for the new classes to separate member and
   non-member function descriptions, removed "class::" from member
   descriptions, added namespace std::chrono around class definitions.
 * Modified time_of_day examples to include sample code producing the
   given output
 * Removed exposition-only members tzdb::next and tzdb_list::head_ that
   are not actually used in the exposition.
 * Modified leap-second example to not require updates each time a
   leap-second is added.
 * Removed exposition-only members from link and leap, merging the
   relevant description into the corresponding accessor.
 * Plus a variety of changes to massage the wording and fix spelling
   and grammar errors, and avoid unfortunate line wrapping.

Author: zygoloid

source/iostreams.tex

@@ -10741,7 +10741,7 @@
   enum class copy_options;
   enum class directory_options;
 
-  using file_time_type = chrono::time_point<@\textit{trivial-clock}@>;
+  using file_time_type = chrono::time_point<chrono::file_clock>;
 
   // \ref{fs.op.funcs}, filesystem operations
   path absolute(const path& p);

source/threads.tex

@@ -27,6 +27,10 @@
 requirements.
 If a template parameter is named \tcode{Predicate}, \tcode{operator()} applied to
 the template argument shall return a value that is convertible to \tcode{bool}.
+If a template parameter is named \tcode{Clock},
+the corresponding template argument shall be a type \tcode{C}
+for which \tcode{is_clock_v<C>} is \tcode{true};
+otherwise the program is ill-formed.
 
 \rSec2[thread.req.exception]{Exceptions}