Refactor node attributes management

[fktn-k]

This PR has modified internal node attributes management.
The latest basic_node class hold a fkyaml::node_type to track the current node value type, and a pair of a fkyaml::detail::anchor_status_t & a uint32_t to reach an anchored node if the node itself is an alias node.
Although node attributes requires 12 bytes (4 bytes each), they can be compressed into 4 bytes using bits, which is what is done in this PR.
Since this is just a refactoring of internal data management, nothing should change from the viewpoint of library users.

Actually this PR is a kind of preparation for per-node serialization style specification (stored with unused bits) like:

• block style or flow style
• single quoted, double quoted or plain string
• decimal, octal, hexadecimal integer
• etc...

Note the above output style specification is not yet implemented, but is planned to be added until the next release.

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• 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.
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