perf: overall improvements

[SukkaW]

What does it do?

• Make some functions pure and move them to the top level
• Avoid some Promise overhead

Avoid nested promise and use as less .then as possible (Every then the promise implementation will push the task into the next tick of the event loop)

• Utilize Array method on non-performance-intensive tasks
• Avoid using new Promise

The author of Bluebird once mentioned new Proimse is slow. And during my recent tests (benchmark suite), it is indeed slower than directly returned Promise, also uses more memory.

How to test

git clone -b perf-refactor https://github.com/sukkaw/hexo.git
cd hexo
npm install
npm test

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.

[coveralls]

Coverage decreased (-0.05%) to 98.135% when pulling 5ea2ff6 on SukkaW:perf-refactor into a2ec6b2 on hexojs:master.