This PR has added the support for reverse iterations over sequence/mapping nodes.
Along the way, the following APIs have been added to basic_node class.

• member types
  • reverse_iterator
    • reverse iterator type, derived from basic_node::iterator
    • documentation link: https://fktn-k.github.io/fkYAML/api/basic_node/reverse_iterator/
  • const_reverse_iterator
    • const reverse iterator type, derived from basic_node::const_iterator
    • documentation link: https://fktn-k.github.io/fkYAML/api/basic_node/reverse_iterator/
• member functions
  • rbegin() and crbegin()
    • returns a (const) reverse iterator to the reverse-beginning element.
    • documentation link: https://fktn-k.github.io/fkYAML/api/basic_node/rbegin/
  • rend() and crend()
    • returns a (const) reverse iterator to the reverse-end element.
    • documentation link: https://fktn-k.github.io/fkYAML/api/basic_node/rend/

Also, the API documentation page for basic_node::iterator and basic_node::const_iterator has been updated to provide more concrete information for visitors.

Visit documentation pages for details.

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.