Add DesugarTests

[Martin Janiczek]

For what should be in them: don't check that it "does nothing". Check the interesting stuff that happens in the desugarExpr function. Eg.:

• Var: finds the module according to the surrounding context (imports and other top level definitions)
• Lambda: gets curried, ie. \a b -> c becomes \a -> \b -> c
• List: keeps the original order