Add toJSON

[yassernasc]

No description provided.

[kasperisager]

This is one of the Node.js features I'd rather leave at "toJSON is not a function". It was from its v0 days.

[yassernasc]

No problem, we can close the PR then and reopen it when needed if that day comes.

[kasperisager]

What I'd actually much rather we do if we were to add buffer.toJSON() is have it return a plain array. That's not only more general but also intentionally breaks compatibility with other Node.js-likes in a way that forces consumers to either a) use a structured schema or b) use the replacer and reviver arguments of the JSON methods. Both options are better and supported in all JavaScript environments regardless of the presence or absence of buffer.toJSON().