Added
* Command-line interface capable of running code-generation and outputting version info
* Internal request schema which gives us all the functionality we could ever need to migrate request bodies forward without any complexity of the prior solution
* `_get_versioned_router` and experimental header routing with it (by tikon93). Note that the interface for this feature will change in the future
Changed
* Renamed `cadwyn.regenerate_dir_to_all_versions` to `cadwyn.generate_code_for_versioned_packages`
* Renamed `cadwyn.generate_all_router_versions` to `cadwyn.generate_versioned_routers`
Removed
* `unions` directory and all logic around it (replaced by internal request schema)
* `FillablePrivateAttr` and all logic around it (replaced by internal request schema)
* `schema(...).property` constructor and all logic around it (replaced by internal request schema)
* Special-casing for code generation of package with latest version using star imports from latest (replaced by internal request schema)