Method to create from `Buf`

[kixelated]

A helper to create a BufList from a Buf would be nice, and would avoid allocating if the provided Buf is a BufList (or similar) with more than 1 chunk.

I think this is how you accomplish it today:

		let mut payload = BufList::new();
		while !buf.chunk().is_empty() {
			payload.push_chunk(buf.copy_to_bytes(buf.chunk().len()));
		}

[kixelated]

Additionally, BufList should implement copy_to_bytes to avoid making a copy if there's only one element.