add tests

[odanoburu]

at least to check a few properties such as the identity on parsing/printing.

maybe use this library? https://github.com/mrkkrp/hspec-megaparsec

[arademaker]

Let us add more ideas here about what tests we can create for the lib...

[odanoburu]

what I mostly did was the roundtrip parse—pretty-print, but that leaves off the diffing functionality

[arademaker]

In long term should be nice have quickCheck