Improve readability, avoid repetition

[fregante]

No description provided.

[sindresorhus]

What is "common usage"? What's being changed here, and why?

[fregante]

All are pretty minor optimizations

• Clarity: The arguments copy is only needed to when multiArgs === true so I avoided its repetition by just grouping by multiArgs instead of errorFirst (seems logical?)
• Clarity: no more let i = 1, results[i - 1]
• Optimization: multiArgs === false && errorFirst === false doesn't need a wrapper, so resolve is passed verbatim as the callback
• Optimization: multiArgs === false && errorFirst === true is probably the most common style and now it's reduced to a single if/else

[fregante]

Admittedly the PR title was a bit sloppy

[sindresorhus]

Thanks. Much better!

I've further simplified the code in 523bad9 by targeting Node.js 6. Any other simplifications you can see?