[basic.string.general] Use (const_)reference for consistency

with other sequence containers

Author: frederick-vs-ja

source/strings.tex

@@ -2131,10 +2131,10 @@
     constexpr const_reference at(size_type n) const;
     constexpr reference       at(size_type n);
 
-    constexpr const charT& front() const;
-    constexpr charT&       front();
-    constexpr const charT& back() const;
-    constexpr charT&       back();
+    constexpr const_reference front() const;
+    constexpr reference       front();
+    constexpr const_reference back() const;
+    constexpr reference       back();
 
     // \ref{string.modifiers}, modifiers
     constexpr basic_string& operator+=(const basic_string& str);