Don't allow mutable references to JSON strings (#1466)

Signed-off-by: Juan Cruz Viotti <[email protected]>

src/json/include/sourcemeta/jsontoolkit/json_value.h

@@ -521,20 +521,6 @@ class SOURCEMETA_JSONTOOLKIT_JSON_EXPORT JSON {
   /// ```
   [[nodiscard]] auto to_string() const noexcept -> const String &;
 
-  /// Convert a JSON instance into a standard string value. The result of this
-  /// method is undefined unless the JSON instance holds a string value. For
-  /// example:
-  ///
-  /// ```cpp
-  /// #include <sourcemeta/jsontoolkit/json.h>
-  /// #include <cassert>
-  ///
-  /// const sourcemeta::jsontoolkit::JSON document{"foo"};
-  /// assert(document.is_string());
-  /// assert(document.to_string() == "foo");
-  /// ```
-  [[nodiscard]] auto to_string() noexcept -> String &;
-
   /// Get a standard input string stream from a JSON string. The result of this
   /// method is undefined unless the JSON instance holds a string value. For
   /// example:

src/json/json_value.cc

@@ -411,11 +411,6 @@ auto JSON::operator-=(const JSON &substractive) -> JSON & {
   return this->data_string;
 }
 
-[[nodiscard]] auto JSON::to_string() noexcept -> JSON::String & {
-  assert(this->is_string());
-  return this->data_string;
-}
-
 [[nodiscard]] auto JSON::to_stringstream() const
     -> std::basic_istringstream<Char, CharTraits, Allocator<Char>> {
   return std::basic_istringstream<Char, CharTraits, Allocator<Char>>{