Simplify union types

[mikegratton]

For union types, the generated code currently throws exceptions when accessing the non-selected member or when changing the discriminator in some instances.

I think these aren't helpful. Changing the discriminator should always succeed. In other words, it should be legal to do

SomeUnionType foo;
foo._d(kCaseBar); // Select another member "bar"
foo.bar().push_back(4); // Then mutate that member

Likewise, accessing non-active members should be allowed. There is nothing technically wrong with first modifying a non-active member, then making that member active later. Users can check the discriminator before serialization to avoid bugs.

[mikegratton]

I've added a pull request to demonstrate what I am proposing #207