`util.toCallback(asyncFn)` - interop of async functions with callbacks.

[benjamingr]

Similar to #5 I'm looking at ways to make the lives of callback users easier with async/await landing. I'm wondering if core should provide a method called toCallback that takes an async function and converts it to a callback returning function. This is similar to asCallback except taking a function:

async function fooP() {
    // some async/await stuff
    // ...
    return "Hello"; 
}
const foo = util.toCallback(fooP);

foo((err, data) => console.log(data)); // hello

The name and where it is are both debatable.

[chrisdickinson]

Does anyone object to making this issue non-blocking for the Promises PR?

[benjamingr]

This issue is orthogonal (and certainly non-blocking) for the promises-in-core PR. It is only about async functions being more ergonomic to callback users.

[chrisdickinson]

Excellent! Did you see callbackify in the existing PR?

[chrisdickinson]

Would you object to discussing a public util.promisify in this issue, or would you prefer to keep that separate?

[benjamingr]

Well, there are different stakeholders so I'd rather keep them separate. It seems like a generally good idea to keep discussions small to include participation. I don't feel very strongly about it though.

[benjamingr]

This landed