Use c8 for code coverage

[GMartigny]

Since node 10.10.0, code coverage is natively supported. The c8 package use this new feature and report a ~300% performance improvement.
Once this feature has live a little and node is more widely used, we should use it instead of nyc.

[GMartigny]

Need bcoe/c8#34 issue to be resolved.

[bcoe]

@GMartigny there is now support for source-maps in c8, might be worth digging into this again.

[GMartigny]

@bcoe Thanks for the heads up. I tried it, but I still have trouble with the coverage value. I've set sourceMap to esm, run c8 ava (which itself require esm) and the result is off.

I'll try to create a minimal reproduction repo and keep you updated.

[GMartigny]

Hi again @bcoe,
You can try a minimal reproduction of my setup for yourself. I may be doing something wrong, but for now this report a 100% coverage when it should not be.