codec: remove unnecessary trait bounds on all Framed constructors

[conradludgate]

Motivation

Minor ergonomics, mostly. To avoid Framed::from_parts(parts) which always reserves 8KiB in the read and write buffers I have been using

fn my_from_parts<T, U>(parts: FramedParts<T, U>) -> Framed<T, U> {
    let mut framed = Framed::with_capacity(parts.io, parts.codec, 0);
    *framed.read_buffer_mut() = parts.read_buf;
    *framed.write_buffer_mut() = parts.write_buf;
    framed
}

It's a minor annoyance that the above function actually needs a T: AsyncRead + AsyncWrite bounds, whereas Framed::from_parts(parts) doesn't.

Solution

Remove the bounds on all Framed constructors.

[ADD-SP]

Thanks!