Add splice and sendfile

[carbotaniuman]

Adds splice and sendfile, as those got added sometime in when I last looked at this and now.

I'm very unsure of all the unsigned to signed conversions and back here, and I'm not really sure why rustix has offsets as unsigned here.

I've uncommented the tests, but strangely enough I don't think that tests actually tests the splice or sendfile path, as those appear locked behind benches. I will add some tests for this.

[sunfishcode]

Rustix uses unsigned offsets to be consistent with std::io::SeekFrom using unsigned, when talking about an offset from the start of a file. That's a debatable choice, but it's the choice that was made.

[sunfishcode]

This looks good to me; it just needs a rustfmt now.

[sunfishcode]

Thanks!