Mapping key/value access in range based for loops

[fktn-k]

This PR adds a new basic_node class API map_items() which allows using iterator member functions, key() and value(), in range-based for loops.
For, since iterator's operator*() returns a reference to a mapping value node, it was impossible to iterate over mapping node referring both to keys and values just as described in the discussion #400.
The map_items() function returns a helper range object which allows accessing those functions in range-based for loops like this:

fkyaml::node mapping = {{"a", 123}, {"b", nullptr}};
for (auto it : mapping.map_items()) {
    std::cout << "key: " << it.key() << "value: " << it.value() << std::endl;
}

// output:
// key: a, value: 123
// key: b, value: null

Furthermore, when this library is compiled with C++17 or better, structured bindings are also usable with mappings.

fkyaml::node mapping = {{"a", 123}, {"b", nullptr}};
for (auto& [key, value] : mapping.map_items()) {
    std::cout << "key: " << key << "value: " << value << std::endl;
}

// output:
// key: a, value: 123
// key: b, value: null

See the API reference page for map_items() for more details.

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