Replace `AsSliceU8` trait with `zerocopy::AsBytes`

[mkroening]

Padding is uninitialized: Glossary - Unsafe Code Guidelines Reference

We do not check for padding, neither is AsSliceU8 unsafe.
zerocopy::AsBytes performs these checks.

[joannejchen]

Hi @mkroening! @scotran and I are part of a Virtualization course at UT Austin, and as part of a project we have to contribute to open source repositories. Can we take up this issue?

[mkroening]

Sure, do you want to do all three issues?

PS: To assign @scotran, he has to be a participant of this issue. Just write anything, then I can add him as well.

[joannejchen]

Yep, all three issues would be great!

[joannejchen]

Hi @mkroening! I was able to switch the VirtioNetHdr (currently reflected in the pull request created) from implementing AsSliceU8 to deriving AsBytes, but I ran into an issue when trying to change from AsSliceU8 to AsBytes for the Cmd and Rsp structs (https://github.com/hermitcore/libhermit-rs/blob/master/src/fs/fuse.rs#L293): #[derive(AsBytes)] is not supported on types with type parameters. Digging around, there seems to be a workaround (google/zerocopy#10), but this would require using repr(transparent), which then cannot be used alongside repr(C) or repr(packed), which would strip padding and possibly cause alignment issue.

I just wanted to touch base first as to whether this would be the desired approach, and if so, I can continue looking into how to derive AsBytes for structs with type parameters.