Unsound transmute in safe method

[shinmao]

CYFS/src/component/cyfs-base/src/objects/chunk.rs

Lines 157 to 159 in 9eaf33f

	pub fn as_object_id(&self) -> &ObjectId {
	unsafe { std::mem::transmute::<&ChunkId, &ObjectId>(&self) }
	}

The safe method as_object_id used transmute to convert &ChunkId to &ObjectId. When we use transmute on reference, we need to ensure the layout (e.g., size and alignment) by ourselves. Even though ChunkId and ObjectId have the same type definition, rust compiler preserves the right to reorder and add padding bytes to the struct of repr(Rust) representation. Suggest to add repr(C) to both struct to make this guarantee.