add io.Lazy

[guillaumearm]

Non-blocking asynchronous flow

io.Lazy allows handlers to have more control on asynchronous flow.

yield an io.Lazy in an handler allow to get promise instead of value, this promise must be yield to be resolved.

e.g.

const { io, handler } = require('handle-io');

const sleep = io(ms => new Promise(resolve => setTimeout(resolve, ms)));

const myHandler = handler(function*() {
  const sleepPromise1 = yield io.Lazy(sleep(1000));
  const sleepPromise2 = yield io.Lazy(sleep(2000));
  const sleepPromise3 = yield io.Lazy(sleep(3000));

  // at this point, all sleep IOs are ran in parallel

  yield sleepPromise1; // waiting for first sleep
  yield sleepPromise2; // waiting for second sleep
  yield sleepPromise3; // waiting for third sleep
});

The important point to note here is that yield sleep(1000) is blocking while yield io.lazy(sleep(1000)) is non-blocking.

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Related to #138.