To-do list

[tshort]

Here are known things that need work:

• [x] Rename functions to support multiple dispatch
• [x] Support recursion
• [ ] Support global variables
• [ ] Support Union types
• [ ] More intrinsics
• [ ] Add a way to override codegen! methods, probably by adding a type parameter to CodeCtx
• [ ] Add a way to intercept function calls to substitute something more "native"