Leading or trailing thunks

[plievone]

It might clarify Readme.md examples if the difference between two kinds of thunks would be made clearer. Currently the thunks are introduced as trailing kind:

function read(...args) {
  return function thunk(cb) {
    // start work, cb(err, result) later
  }
}

but functions from thunkify start the work already on the leading edge:

function read(...args) {
  // start work, save result later
  return function thunk(cb) {
    // cb(err, result) or when result ready
  }
}

This difference helps to understand when work can start concurrently (in the event loop sense) even before yielding.

Currently the frontpage examples mix both styles, confusing where the concurrency comes in each case. And the example with "3 concurrent requests at a time" and "9 concurrent requests" won't do what it says, because there are those sequential yields in the *results() generator, whatever style the thunks are.

[navaru]

It think this is an implementation detail in thunkify, a thunk could be expressed the same, but in the case of thunkify, it's more like an optimization, in some cases by the time the generator calls .next() the asynchronous function may be done.