view: prefer std::next over std::advance

next is slightly less likely to trigger issues dereferencing a temporary
(since we aren't producing an lvalue).

string.hpp

@@ -80,9 +80,7 @@ namespace util {
 
             iterator operator+ (int count)
             {
-                auto next = *this;
-                std::advance (next, count);
-                return next;
+                return std::next (*this, count);
             }
 
             const range_type& operator* (void) const

view.hpp

@@ -356,18 +356,14 @@ namespace util {
         constexpr auto&
         operator[] (size_t idx)& noexcept
         {
-            auto it = begin ();
-            std::advance (it, idx);
-            return *it;
+            return *std::next (begin (), idx);
         }
 
         //---------------------------------------------------------------------
         constexpr auto&
         operator[] (size_t idx) const& noexcept
         {
-            auto it = begin ();
-            std::advance (it, idx);
-            return *it;
+            return *std::next (begin (), idx);
         }
 
     private: