feat: add benchmark with lots of files

[zkat]

based on https://github.com/sciolist/fcopy/blob/c0ded8b82bbaa3f4cdb0a67e951eb3f8b0f97fb7/perf/project/alottafiles/package.json

Similar to #10 -- this is intended to provide a more realistic view of what a modern app will often look like. This specific package.json was the one used to benchmark Yarn's new fs.copyFile() feature, which sped things up quite a bit.

[zkat]

Oh yeah, probably want consent from @sciolist before actually merging this one. I'd previously gotten approval from @corbinu for #10, so I'm less worried about that one.

[zkochan]

Should I run this on a COW filesystem? I'll have to check if my Windows/Linux support COW

[zkat]

@zkochan I feel like that's an extra dimension to the benchmarks, and CoW is still not exactly widespread on OSX and a lot of Linux systems. I think it's still worth keeping a non-CoW version around (or just do that by making the benchmarks be per-platform, and provide information on what each one does).

[BYK]

It is important to note Node version too. copyFile is not available in earlier versions of node which makes a big difference.

[sciolist]

@zkat Absolutely fine with me.