#167 Use std::map as the default mapping node type

[fktn-k]

Currently, mapping nodes preserves the order of insertions by using ordere_map class.
However, I noticed that YAML specification 1.2.2 defines mapping as “an unordered association of unique keys to values“.
(See for more information here.)

To follow the specification, basic_node class will be changed to use std::map as mapping container class by default.
Note that std::unordered_map is not supported since it does not support the use of incomplete types on template instantiation.

This change will change the serialization/deserialization results, but bring the fkYAML library more efficiency in performance.

Lastly, the ordered_map class will still be available as an optional mapping container type for customization(, rather than using a sequence of mapping node as the specification suggests).

#include <fkYAML/node.hpp>
using ordered_node = fkyaml::basic_node<std::vector, fkyaml::ordered_map>;

[coveralls]

coverage: 100.0%. remained the same when pulling c1fce5e on feature/167_make_mapping_node_unordered_by_default into e5c30c3 on develop.