Consider adding libuv-based backend

[clue]

This likely depends on libuv-based event loop implementation reactphp/event-loop#17.

Once this is done, see reactphp/reactphp#213 for initial starting point on filesystem abstraction.

[steverhoades]

I initially started working on a solution right around the time @WyriHaximus was working in the filesystem. Unfortunately I haven't had any free time in the last several months to circle back to it.

Here: https://github.com/steverhoades/ReactLibuvFilesystem

The I did some considerable work on the php-uv extension to bring it up to libuv 1.0 as well, but as far as I know that still hasn't been merged.

[WyriHaximus]

@steverhoades is the php-uv extension with your changes stable? If it is, we can continue your work on the adapter if you don't mind.

I have a working uv adapter (PHP7 only, so waiting for #67), which I only need to add tests for.

[WyriHaximus]

@CharlotteDunois awesome!