New one that focused on performance.

[ifsnow]

First of all, thank you for giving me an opportunity to do something fun.

I'm impressed with the concept of co module.
so, I tried making a new implementation that focused on performance.
I think it's worth doing such a micro-optimization. Not all, but at least in the core modules.

Here's my working branch.

These are my goals.

* Run faster.

• benchmark - co() / test code

  • Node 4.4.7 : 8% faster
  • Node 6.9.1 : 12% faster

• benchmark - co.wrap() / test code

  • Node 4.4.7 : 3% faster
  • Node 6.9.1 : 8% faster

• benchmark - koa / test code

  • Node 4.4.7 : 15% faster
  • Node 6.9.1 : 32% faster

* Keep compatibility.

It's fully compatibility with original co. Passed all test cases.

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I'm thinking of providing it for the public with a different module. for example, co-booster.
but, I'm not sure it would be helpful to them as much as I think. :)

Please let me know if you have any concerns or advice.