Remove the TaggedValue enum

[Keavon]

Discussion here: https://discord.com/channels/731730685944922173/860830913390575646/1412529955841245345 where @TrueDoctor wrote:

We would need a self describing serialization format which does store the type information but if we want arbitrary compound types to work we will always have to do some form of manual deserializaiton

something like this: https://github.com/dtolnay/typetag

… this crates uses a similar link time hack to generate a list of all possible implementations for deserialization as we use for our node definitions

So it should be possible to remove tagged values (or we could at least experiment with that), it's just a question of if that is useful to us rn and if we should prioritize this. And I don't think we should do that since tagged values work just fine for us rn