Add the ability to talk about Plutus versions

[WhatisRT]

We do have a Language type, but it's abstract and don't have access to inhabitants like PlutusV1. There are at least two options to resolve this:

• Change Language to be a regular sum type
• Add new fields to PlutusStructure, such as PlutusV1 : Language

The second option has the benefit of being more flexible. Language could contain things that we don't need to talk about or once we have different versions of the spec we wouldn't need to translate between different types here. If there are no downsides, that option is preferred.