IoSlice and IoSliceMut

[sunfishcode]

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 and IoSliceMut, though for my project, I was able to just disable vectored I/O in no_std builds.

[bbqsrc]

It was hard and I didn't feel like putting in the work since I don't know the real world use cases. If someone wants to patch it in, they may be my guest. 😄