Update codegen to support Sysl union type.

[ericzhang6222]

This is a simple and quick fixing as #95 is a blocker issue, now it can transfer Sysl union type too. For example, transform

!type Cat:
        id <: string 
        age <: int?:
            @json_tag = "age"
        hunts <: bool?:
            @json_tag = "hunts"

!type Dog:
        id <: string
        bark <: bool?:
            @json_tag = "bark"
        breed <: string?:
            @json_tag = "breed"

!union PetA:
        Cat
        Dog

to

// PetA can be one of following types in runtime:
// Cat
// Dog
type PetA interface {
    // isPetA is identifier method
    isPetA()
}

// isPetA identifies Cat is instance of PetA
func (i Cat) isPetA() {
}

// isPetA identifies Dog is instance of PetA
func (i Dog) isPetA() {
}

It can be enhanced still, like adding util method to transform instance of Pet to Cat or Dog.

[ericzhang6222]

I see an issue lurking over here, as per my understanding, both Cat and Dog can behave as Pet during runtime as they are part of a union(both implement Pet interface {}). But essentially in the generated code the method set associated with Pet interface {} is empty, which means even a struct like type struct Empty {} can be a Pet. Please add a method to the Pet interface {} and implement the same in the structs Cat and Dog.

Fixed.