Update to n-api

[fanatid]

node@8 not supported anymore, it's good switch to n-api, because it's not require adjust cpp code on each v8 change (as NAN).

Also:

• added prebuilds (prebuildify) with GitHub Actions
• removed second pure js implementation
• slightly changed api, now not require Buffer
• only 1 dependency -- elliptic, except node-addon-api/node-gyp-build for building addon

[kumavis]

Fixes #161 🙌