[API BREAKING] Remove conversion from weekday to unsigned

* There has been a great deal of anguish over the encoding of
  weekdays:  whether [0, 6] maps to [Sunday, Saturday] or
  [1, 7] maps to [Monday, Sunday].  This commit attempts
  to address that issue, but will break a small amount of
  code at compile-time.  See below on how to fix that.

* The weekday constructor used to accept [0, 6] to represent
  [Sunday, Saturday].  It now accepts [0, 7] to represent
  [Sunday, Saturday] with both 0 and 7 mapping to Sunday.

* The conversion from weekday to unsigned has been removed.

* To convert a weekday to unsigned replace:

      auto u = unsigned{wd};

  with:

      auto u = (wd - Sunday).count();

  This maps [Sunday, Saturday] to [0, 6], which is the
  C/POSIX mapping.  If you prefer the ISO mapping
  ([Monday, Sunday] -> [1, 7]), then do:

      auto u = (wd - Monday).count() + 1;

test/iso_week/weekday.pass.cpp

@@ -171,7 +171,7 @@ test_with_date_weekday()
     auto constexpr d1 = iso_week::sun;
     static_assert(unsigned{d1} == 7, "");
     auto constexpr d2 = date::weekday{d1};
-    static_assert(unsigned{d2} == 0, "");
+    static_assert(d2 == date::Sunday, "");
     auto constexpr d3 = iso_week::weekday{d2};
     static_assert(unsigned{d3} == 7, "");
 }