refactor using Nan

[hackmod]

• refactor multihashing.cc using Nan without much impact on the current structure.
• fix deprecated methods (Uint32Value() like wrapper methods are removed from nodejs v12. #80)

Please see also travis results against node v12 https://travis-ci.org/github/hackmod/node-multi-hashing/builds/715330915

[github-actions[bot]]

:tada: This PR is included in version 1.0.0 :tada:

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