Create Algebraic Data Type for Choices to replace Name

[peter-michalski]

This issue is for managing the suggestion from @JacquesCarette comment.

I have started to update Choices.hs in cfa8ae4b2157080429b1117e22cc4d6393c382c2, adding the following data type to start:

data DesignChoice = GetInputChoice deriving (Show)

designChoiceStr :: DesignChoice -> String
designChoiceStr GetInputChoice = "get_input"

I have added Either's into FunctionCall.hs to manage this change.

@JacquesCarette please review and let me know if the above data type is suitable.

[JacquesCarette]

See comments left on the commit itself.