Add Parse<->Print isomorphism tests

[GuiltyDolphin]

@dorchard As discussed, this is a suggestion to add tests to help ensure consistency of the parser/printer.

Gen->Print->Parse

We could use QuickCheck to generate syntax trees, then 'pretty' show them in a canonical way and parse them. We would expect the parsed syntax trees to be the same (modulo some information such as position) as the original syntax tree.

File->Parse->Print

Assuming we had example programs in a 'pretty' canonical form, we would expect parsing the files, then printing them, to give the same text as the original file.