Always run limited functions asynchronously

[copperwall]

This modifies the generator function returned from plimit to always
run functions passed to limit asynchronously.

This is done by modifying the enqueue method to always push the function
provided to the generator function onto the queue and then scheduling a
function to run asynchronously to dequeue and run that function.

I thought using using Promise.resolve().then() was a nice way to
schedule that dequeuing function asynchronously because it wasn't too
node-specific or browser-specific, but I'm happy to do something else
like preferring process.nextTick and then falling back to setTimeout
if process.nextTick isn't available.

Closes #22

[sindresorhus]

I thought using using Promise.resolve().then() was a nice way to
schedule that dequeuing function asynchronously because it wasn't too
node-specific or browser-specific, but I'm happy to do something else
like preferring process.nextTick and then falling back to setTimeout
if process.nextTick isn't available.

It really depends on how much breathing room we want to give it. setImmediate runs in the next even loop tick. nextTick run at the end of the current event loop tick. And Promise.resolve() runs in the next macro-tick (which is much shorter). Since we don't know the use-case, I think Promise.resolve() is fine and it also works in any environment.

[copperwall]

Hey @sindresorhus, it looks like the node 6 test failed from using async. I know node 6 and 8 are EOL'd, but I don't want to break compatibility if this library is still targeting those versions. I can revert the change or bump the engine requirement, either way.

[sindresorhus]

I plan to target Node.js 10 after this PR, so just ignore anything lower.

[sindresorhus]

Thanks for fixing this 🙌

[sindresorhus]

This is now published: https://github.com/sindresorhus/p-limit/releases/tag/v3.0.0