This PR adds the following changes:

• The following legacy container types are added to the basic_node template class as its member types.
  The iterator and const_iterator of basic_node refer to them when instantiating their types.

  name	description	associated type
  value_type	type of elements of a container node	basic_node
  reference	type of reference to an element of a container node	basic_node&
  const_reference	type of const reference to an element of a container node	const basic_node&
  pointer	type of pointer to an element of a container node	basic_node*
  const_pointer	type of const pointer to an element of a container node	const basic_node*
  size_type	type to represent container node sizes	std::size_t
  size_type	type to represent differences between elements of a container node	std::ptrdiff_t

• basic_node's iterator and const_iterator objects are now compatible regarding:
  • copy construction/assignment
    • const_iterator objects still cannot be assigned to or used for constructing iterator objects so the const-ness of const_iterator is not casted away.
  • relational comparisons
    • operator==, operator!=
    • operator<, operator<=, operator>, operator>=
• New member functions cbegin() and cend() are added to basic_node.
  They always return a const_iterator object to the first and past-the-last element respectively just like those provided in STL container types.

The test suite and the API documentation contents have also been updated according to the above changes.

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.