Refactor tamarin-prover-export

[arcz]

Same as #621 but for the export part.
Is there a reason for it being a separate package? It's just two files, it would be easier to maintain if it was just part of the main package.

[rkunnema]

Hi!

@yavivanov is introducing substantial changes to Export.hs at the moment, so I would keep this PR on hold until I get confirmation from him how well this can merge with this (functional) changes. Maybe it is better to apply the linter again after merge.

For some choice I am unsure: substituting pure for return (discussion at https://stackoverflow.com/questions/32788082/difference-between-return-and-pure). So I'd prioritize a smooth merge over a uniform use of one or the other.

Finally, export is in a separate package because it enforces better encapsulation. Moreover, Export.hs has now reached the size where it ought to be split into several files. I don't see much of a downside to it.

[arcz]

No problem, feel free to merge whatever you need I'll just rebase after that.