[v3] Export Pharo objects to source code text files.

[samWson]

Source code is not written in text files in Pharo. Instead it is all in an image file which is essentially a snapshot of object memory.

To write exercises for the website we need to serialize Pharo objects as human readable text files. This is something we already do for v2. I'm going to rewrite just the code I need and add it to the pharo-smalltalk part of this repo.

The outcome of this issue is that we can write exercises in the Pharo IDE and have the exercise text files we need in the repo.

[samWson]

This pull request to the V2 repo puts the existing exercise generator code under test: #426

With that done I can duplicate most of it's functionality for a V3 exercise generator. I'll then move common code into an abstract super class.