Refactor to allow internal Tonel generated without files

[bencoman]

We are writing a GUI for Exercism.io to submit solutions directly from Pharo to their web server,
so we need to generate Tonel files as strings without touching the file system.
Refactor to facilitate this.
[EDIT: I forgot to mention, I'll submit a PR for this soon]

[jecisc]

+1

I would also like it to display the code of the Spec Examples next to them in a tab.

[jecisc]

@bencoman Do you want to export a full package or just a class?

I don't see how you'll do to export a full package without files since it needs to generate multiple files. (Note that you can do it without "real" files with a MemoryFileSystem).

If it's to export only a class, I'll propose a PR.

[jecisc]

@bencoman Can you check if this fixes what you want?

#56

[bencoman]

@samWson, @macta fyi...

I've copied this from the PR for a more permanent record.
@jecisc, For Exercism we want to export a whole package-tag i.e. under our 'Exercism' placeholder package we have one exercise per package-tag. Here is the Tonel changes that facilitated that.

If you add that to a fresh system it can be used like this...

packageTag := (RPackageOrganizer default packageNamed: 'AST-Core') classTagNamed: 'Nodes'.
tonelMap := TonelWriter new mappedSnapshot: packageTag snapshot.

Then tonelMap is iterated to send it across the network.