Move object keys into the value when parsing

json_sax_dom_parser::key() copied the key from the lexer's buffer into the
object. The lexer starts every token with an empty buffer (reset()), so the
key can be moved instead, which saves an allocation and a copy for every key
that does not fit into the string's inline buffer. json_sax_dom_parser
already moved string values the same way. ordered_map has no rvalue
operator[], so ordered_json still copies keys.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/input/json_sax.hpp

@@ -411,8 +411,9 @@ class json_sax_dom_parser
         JSON_ASSERT(!ref_stack.empty());
         JSON_ASSERT(ref_stack.back()->is_object());
 
-        // add null at the given key and store the reference for later
-        object_element = &(ref_stack.back()->m_data.m_value.object->operator[](val));
+        // add null at the given key and store the reference for later; the
+        // key is moved, as the lexer starts the next token with reset()
+        object_element = &(ref_stack.back()->m_data.m_value.object->operator[](std::move(val)));
         return true;
     }