Further simplify operator++

Remove unnecessary clearing of to_increment_ and clean up logic.

Author: ednolan

include/beman/utf_view/to_utf_view.hpp

@@ -203,15 +203,12 @@ class exposition_only_to_utf_view_impl {
     constexpr void exposition_only_advance_one() // @*exposition only*@
       requires std::forward_iterator<exposition_only_iter>
     {
-      if (buf_index_ + 1 < buf_last_) {
-        ++buf_index_;
-      } else {
+      ++buf_index_;
+      if (buf_index_ == buf_last_) {
+        buf_index_ = 0;
         std::advance(base(), to_increment_);
-        to_increment_ = 0;
         if (base() != end()) {
           read();
-        } else {
-          buf_index_ = 0;
         }
       }
     }