Rework codebase

[ffaf1]

A mild cleanup of the codebase to bring it up to date. Includes:

• fix warnings;
• add convenience functions;
• add a minimal testbase;
• fix whitespace;
• update .cabal file.

I kept the commit small so if there is something you don't want to see, you can refuse it.
Once this is approved, I can squash it all in one.

[ffaf1]

@portnov Do you further explanations on this?

[portnov]

I hope to be able to look through this next couple of weeks.