so it turns out that everything is always Bytes

[hawkw]

Currently, the http_box::BoxBody type erases both the type of the
http_body::Body and the Body::Data associated type (to
Box<dyn Buf + Send>). However, it turns out that erasing the Data
type isn't actually necessary: all Body types in Linkerd use
Bytes as the Data type.

This branch changes BoxBody to just require that B::Data is Bytes,
and always produce Bytes as its data type. This will enable a much
more performant implementation of request body buffering for POST
retries (linkerd/linkerd2#6130). Rather than copying all the bytes from
a dyn Buf Data type into an additional buffer, we can just clone the
Bytes, increasing the reference count for that buffer, and return the
original Bytes. This means we don't need to perform additional memcpys
to buffer bodies for retries; we just hold onto the already-allocated
Bytes buffers.

The cloned Bytes can then be stored in a vector, and if we have to
write them out again for a retry, we can do one big vectored write :)

[hawkw]

Actually, upon further consideration, I think we don't want to do this. Making all the buffers Bytes means we can't do the "one big vectored write thing", and would instead have to stream the body chunks out one Bytes at a time.

Instead we should use Buf::copy_to_bytes, which i had forgotten does a shallow clone ref bump when the Buf is a Bytes.

nevermind lol