#240 Support explicit block mappings & non-scalar nodes as mapping keys

[fktn-k]

In this PR, explicit block mappings and sequence/mapping keys have been supported in the deserialization feature.
Also, new unit test cases have been added to ensure the newly supported features work as expected.

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Pull Request Checklist

Read the CONTRIBUTING.md file for detailed information.

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

Please don't

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[coveralls]

coverage: 100.0%. remained the same when pulling 3bcb93d7738245995b5d990ddf3e6aff5af1d9b7 on feature/240_support_explicit_mapping_keys into ffe16ae924211f111a2bf94f14a8b57ac60149b9 on develop.