Avoid duplicate hash computation during object creation

[Sebastian Thiel (Byron)]

Fixes #2516

Tasks

• refackiew
• commits with decent changelog entries

[Jon Ross-Perkins (jonmeow)]

Thanks! This looks good to me, just a couple questions about the Write trait changes (TBH I'm really just comparing with the non-known_id variants)

[Jon Ross-Perkins (jonmeow)]

Should this mirror how write_buf calls write_stream? i.e.:

Suggested change

	let _ = id;
	self.write_buf(object, from)
	self.write_stream_with_known_id(id, object, from.len() as u64, &mut from)

It looks like places like cache.rs omit write_buf, so this kind of call might let them omit write_buf_with_known_id similarly (though looking at the loose/write.rs changes which do provide write_buf, perhaps cache.rs should actually be forwarding write_buf?)

[Jon Ross-Perkins (jonmeow)]

Is it right to be discarding the id here (versus not providing a default implementation)? Maybe this is appropriate for backwards compatibility; if that's the intent, maybe worth a comment?

Suggested change

	let _ = id;
	// Provide a default implementation which discards the id.
	let _ = id;

[Sebastian Thiel (Byron)]

Jon Ross-Perkins (@jonmeow) Thanks so much for taking a look!

I haven't looked at the code at all yet, but when I do I will particularly look at the places you are highlighting.