N-API support

Hi @lovell ,
I worked on this native addon and I ported it to **[N-API](https://nodejs.org/dist/latest-v8.x/docs/api/n-api.html)**
You can find my work here: [https://github.com/NickNaso/64/tree/napi](https://github.com/NickNaso/64/tree/napi)
If you want experiment with N-API you can create a branch and call it "napi" so I can execute the PR on it.
After that we can iterate to over it and at the end publish a tagged version of **64** like reported here: https://nodejs.org/en/docs/guides/publishing-napi-modules/

Start with the linked napi branch and the Node.js N-API documentation to understand the proposed port of this native addon. Compare the branch with the current 64 implementation, then verify that the addon works through N-API and can be published as a tagged version.

Written by the indexing model from the issue text.