Introduce new cgp-field constructs

[soareschen]

• Introduce the product type constructs Cons and Nil.
• Introduce the sum type constructs Either and Void.
• Introduce the Field type for tagged field value.
• Introduce the Product! macro for building product types.
• Introduce the product! macro for building product expressions.
• Introduce the Sum! macro for building sum types.
• Change the symbol! macro to generate product type of Char using Cons and Nil.