Only represent references between AST structures with IDs

[fitzgen]

I'm currently making a first pass over this crate's API to make it id-based similar to walrus (it is currently "physical" in that you have to maintain the index references yourself, similar to parity-wasm).

My first pass will represent refs between AST things as essentially enum ThingRef { Id(id), Name(String), Index(u32) } and then there will be a validation/canonicalization pass that ensures everything is an Id.

However, we would like to only have Ids, and to do that we need to add some validation into the middle of parsing with lalrpop's fallible actions (lalrpop/lalrpop#387).