Remove type synonyms completely in desugaring
Currently, type synonyms remain in data constructor declarations (while they get properly replaced in signatures). Concerning that FC should not contain type synonyms anymore, desugaring should get rid of them.
Note that the dictionary transformation performs this task at the moment. However, it is merely a side effect and does generally not belong in that phase. Moreover, having no type synonyms at all after desugaring simplifies all subsequent phases (including the dictionary transformation that currently has to deal with synonyms).