More diff friendly pretty printing of cabal files
Now the 'fieldGet' function of 'FieldDescr' does the whole pretty printing of the field. Previously only the values have been pretty printed by 'fieldGet' and the name of the field with the colon have been printed in the 'PrettyPrint' module. But this separation made it more difficult to handle the pretty printing of fields differently, because some fields should be just printed in one lines and others - having several values - should be printed nested with each value on a new line. This difference in the printing of the fields is now handled by the 'FieldDescr' constructor functions in 'ParseUtils'. Now the 'listField' and 'commaListField' functions create 'FieldDescr' that nest their values and all other functions create one line for the whole field.
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