Further simplify operator++ and operator--

Remove unnecessary clearing of to_increment_, clean up logic, remove
separate overloads for advance_one()

Author: ednolan

include/beman/utf_view/to_utf_view.hpp

@@ -201,31 +201,19 @@ class exposition_only_to_utf_view_impl {
     /* PAPER */
 
     constexpr void exposition_only_advance_one() // @*exposition only*@
-      requires std::forward_iterator<exposition_only_iter>
     {
-      if (buf_index_ + 1 < buf_last_) {
-        ++buf_index_;
-      } else {
-        std::advance(base(), to_increment_);
-        to_increment_ = 0;
+      ++buf_index_;
+      if (buf_index_ == buf_last_) {
+        if constexpr (std::forward_iterator<exposition_only_iter>) {
+          buf_index_ = 0;
+          std::advance(base(), to_increment_);
+        }
         if (base() != end()) {
           read();
-        } else {
-          buf_index_ = 0;
         }
       }
     }
 
-    constexpr void exposition_only_advance_one() // @*exposition only*@
-      requires (!std::forward_iterator<exposition_only_iter>)
-    {
-      if (buf_index_ + 1 == buf_last_ && base() != end()) {
-        read();
-      } else {
-        ++buf_index_;
-      }
-    }
-
     constexpr exposition_only_utf_iterator& operator++() requires(OrError)
     {
       if (!exposition_only_success()) {
@@ -262,7 +250,7 @@ class exposition_only_to_utf_view_impl {
     {
       if (!buf_index_)
         read_reverse();
-      else if (buf_index_)
+      else
         --buf_index_;
       return *this;
     }