feat(ts-gen): typeguard helpers for unions

[andrewmd5]

this change introduces helper functions that allow you to identify the underlying type of a union in a way that populates information to the type system.

const test = Test.fromA({hello: "world"});
if (test.isA()) {
    // typescript knows that 'value' is 'A'
    console.log(test.value.hello)
   // it also knows that 'data' is { discriminator: 1, value: IA }
    console.log(test.data.value.hello)
}