Add trait ExactSizeBuf as well as corresponding implementations

[regexident]

In many scenarios it is desirable to use abstract traits, such as Buf and BufMut in one's API, rather than explicitly use Bytes and BytesMut (let alone bare &[u8] or &mut [u8]).

In encoding/decoding scenarios an also rather common requirement is to be able to obtain the length of a buffer. Unfortunately neither Buf nor BufMut provide a fn len(&self) -> usize method to do so.

This PR aims to fill this gap without introducing a breaking change by adding a ExactSizeBuf trait alongside several implementations for types provided by this crate.