Closes #863

• The entirety of "runtime" codegen is now following a strict MVC model, and the source code is rendered from the same model object. We should be able to further improve/refactor this model object in Migrate the parser to the language definition v2 #638
• Complex model construction can be strongly-typed in Rust, while rendering is left to templates, as they are much easier to read and modify, and also have powerful macros/imports/callbacks, etc...
• I also (partially) improved the DX when working with templates in terminal, by rendering their errors as an ariadne report, with best-guess source location in the template. A temporary workaround waiting for error locations and IDE diagnostics support Keats/tera#885
• Now both stubs and output crates are rendered from the same source templates, removing a few internal hacks and the need for ![allow(dead_code)] and ![allow(unused)] everywhere (will follow up on this cleanup separately).
• We no longer have to maintain explicit paths and copy files around in the code generator. All templates generate their output file relative to their location, which means we can split and refactor code into as many templates/sub-modules as needed, following standard rust idioms.