First version of an efex bytestream decoder

The slightly unusual source code structure is because this decoder reuses code from the online software to avoid having two versions of code that are designed to do the same thing. For this reason the structure of the source code is deliberately laid out how it appears in the online software, which will make it easier to maintain as changes to the decoder take place (we will change them in the online sw and then copy over the relevant files to the offline sw).

Jira: ATR-25493