This PR has added two new enum classes, node_type and yaml_version_type.
They are added to replace existing fkyaml::basic_node::node_t and fkyaml::basic_node::yaml_version_t type aliases whose aliased types are actually declared in the fkyaml::detail(private) namespace but exposed to library users.
The exposing of those enum classes as public APIs obscures the boundary between the fkyaml(public) and fkyaml::detail(private) namespaces, which might also cause unexpected backward incompatibilities.

To resolve the above issue, this PR has introduced the above two new enum classes, adding some APIs and deprecating some other APIs which uses the mentioned type aliases:

Although all the deprecated APIs still work as before except for compile-time deprecation warnings, it's highly recommended to replace those usages with new APIs since they're planned to be removed in version 0.4.0.
See the Deprecation notes in each deprecated API reference page for the migration guide.

Pull Request Checklist

Read the CONTRIBUTING.md file for detailed information.

• Changes are described in the pull request or in a referenced issue.
• The test suite compiles and runs without any error.
• The code coverage on your branch is 100%.
• The documentation is updated if you added/changed a feature.

Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
• Please refrain from proposing changes that would break YAML specifications. If you propose a conformant extension of YAML to be supported by the library, please motivate this extension.
• Please do not open pull requests that address multiple issues.