Add the length field for Buffer

[HaoYang670]

Which part is this question about
So far, the Buffer doesn't have its own length, which means you can't slice on data with the length you want.:

pub struct Buffer {
    /// the internal byte buffer.
    data: Arc<Bytes>,

    /// The offset into the buffer.
    offset: usize,
}

This is an old question, and related to some bugs (such as #807), and some performance issues (such as #2510)
I reopen this question because we don't reach a consensus.

Personally, I hope Buffer could behave like &[u8] or Arc<Vector>, but we need more discussion.

Additionally, I hope we could reach a consensus, for example:

• We should add the length field, because ...
• we don't need to add this field, because ...

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Additional context

[tustvold]

I would suggest lifting the length out of Bytes onto Buffer, this way we avoid redundant information

[HaoYang670]

Thank you, @tustvold. I will try your suggestion and file a draft PR. (maybe need more help from you)