Codec

This crate provides a trait Codec and two (initial) implementations, Cbor and CborEncrypted.
A Codec value is a convenient abstraction over the lower level serialisation and crypto functions in streambed.

A Codec is also closely associated with a CommitLog and a Topic since values stored on the commit log under a given topic will all be encoded the same way.

The type LogAdapter is provided to wrap a Codec, CommitLog and Topic. A LogAdapter carries a type parameter for the decoded log values. It can be viewed as a typed counterpart to CommitLog

The produce method on LogAdapter accepts a typed value, encodes it and appends it to the log.
The history method returns a Stream of typed values from the commit log.