This PR introduces `EarlyCtx` and `LateCtx` in place of the old `CodegenCtx`, Early passes operate at the AST level while generators and other late passes operate on the schema.
It will also replace the confusing `RType` name with something more idiomatic ~~(open for suggestions, I haven't found a good name yet)~~ I've named it `AstType` and dropped the `R` prefix for `REnum` and `RStruct`.
There are some qualities of life improvements too, Things like `to_type_elide` can be used to simplify the code.
Related to #4442 (and can potentially mark it "close as fixed").
