Add support for bytemuck

[arturoc]

Bytemuck allows to cast types into other types and to get the byte representation of a type or a slice or Vecof that type. With this change for example one can do:

let guids = vec![GUID::rand();100];
std::fs::write("guids.bin", bytemuck::cast_slice(&guids))?;

To write several GUIDs as binary data to a file or other stream.

[kurtlawrence]

Thanks for the PR!