IoSlice and IoSliceMut

Thanks for core2! I was able to port a `std::io`-using codebase to it very easily.

The one thing I needed that was missing was [`IoSlice`](https://doc.rust-lang.org/stable/std/io/struct.IoSlice.html) and [`IoSliceMut`](https://doc.rust-lang.org/stable/std/io/struct.IoSliceMut.html), though for my project, I was able to just disable vectored I/O in `no_std` builds.