Add json{::partial,}::scan() in the likes of the scanf() API

include/trial/protocol/core/detail/lightweight_test.hpp

@@ -226,6 +226,7 @@ void test_all_with_impl(FormattedOutputFunction& output,
 
 #define TRIAL_PROTOCOL_TEST BOOST_TEST
 #define TRIAL_PROTOCOL_TEST_EQUAL BOOST_TEST_EQ
+#define TRIAL_PROTOCOL_TEST_NE BOOST_TEST_NE
 #define TRIAL_PROTOCOL_TEST_THROWS BOOST_TEST_THROWS
 
 #define TRIAL_PROTOCOL_TEST_THROW_EQUAL(EXPR, EXCEP, MSG)               \

include/trial/protocol/json/detail/error.ipp

@@ -58,6 +58,9 @@ public:
 
         case insufficient_tokens:
             return "algorithm used requires more tokens than available";
+
+        case field_not_found:
+            return "request field(s) not found";
         }
         return "trial.protocol.json error";
     }

include/trial/protocol/json/error.hpp

@@ -35,7 +35,8 @@ enum errc
     expected_end_array,
     expected_end_object,
 
-    insufficient_tokens
+    insufficient_tokens,
+    field_not_found
 };
 
 const std::error_category& error_category();