Allow reading/writing tagged tuples more naturally

[z-silver]

Right now, the only way to read or write a tagged tuple (with more than just a single payload) is by mapping it to a Zig tuple that has an enum as the first item. Although this works, it doesn't play nice with the type system. Ideally, we would rather map a tagged tuple to a tagged union where the payload is a tuple or an array (but not a slice).