[string.view.template] Change the reference, pointer etc typedefs in string_view to use value_type rather than charT (#1416)

This is a cut down version of #141, to ensure that it is editorial

Author: mclow

source/strings.tex

@@ -4709,10 +4709,10 @@
   // types
   using traits_type            = traits;
   using value_type             = charT;
-  using pointer                = charT*;
-  using const_pointer          = const charT*;
-  using reference              = charT&;
-  using const_reference        = const charT&;
+  using pointer                = value_type*;
+  using const_pointer          = const value_type*;
+  using reference              = value_type&;
+  using const_reference        = const value_type&;
   using const_iterator         = @\impdefx{type of \tcode{basic_string_view::const_iterator}}@; // see \ref{string.view.iterators}
   using iterator               = const_iterator;@\footnote{Because \tcode{basic_string_view} refers to a constant sequence, \tcode{iterator} and \tcode{const_iterator} are the same type.}@
   using const_reverse_iterator = reverse_iterator<const_iterator>;