Emscripten support

[xzfc]

The target wasm32-unknown-emscripten is considered both wasm32 and unix, so the current master branch does not compile with multiple definition errors. This PR fixes this. Some of the tests checking file mtimes and symlinks still fail on Emscripten, this could be addressed later.

Relies on alexcrichton/filetime#88.

[alexcrichton]

I'm a bit worried about the growth of the usage of #[cfg] here, while windows/unix is pretty simple adding windows/unix/wasm/not-emscripten is getting a bit much. Would you be willing to integrate the cfg-if crate to clean up these definitions?

[alexcrichton]

Thanks!

[atilag]

I'm trying to port a project to wasm32-unknown-emscripten that has this crate as a transient dependency. Willing to take over this and finish the emscripten port just in case @xzfc or @alexcrichton don't have the time to do it :)

[atilag]

I've been taking a look at the PR, and it's becoming challenging to read between different if #[cfg()] / else. For short functions it's not a problem, though. Cognitive Complexity increases significantly with that extra indentation level :)