N-API support

[NickNaso]

Hi @lovell ,
I worked on this native addon and I ported it to N-API
You can find my work here: 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/

[lovell]

Hello again, thanks for this Nick, like with your similar improvements for farmhash at lovell/farmhash#19 we can merge this when prebuild supports N-API too.

[NickNaso]

Hi @lovell I want only notify that last week prebuild added the support for N-API https://www.npmjs.com/package/prebuild#n-api-considerations.