Cadwyn

Fixed

* `fastapi.Response` subclasses with non-null bodies and 500 response causing the response to not get returned
* `fastapi.Response` subclasses had invalid content length if migration affected it

Changed

* Rewritten header routing logic and structure to support the full feature set of FastAPI

Fixed

* Modules and enums from head versions not being detected and thus causing errors

Changed

* Header router is no longer reliant on the API version header -- now it simply takes the API version from the `VersionBundle.api_version_var`, thus making it easy for someone to extend header routing and set their own rules for how the default version is chosen

Fixed

* Previous version introduced a minor breaking change: if any old users depended on the pure `generate_versioned_routers` interface, their work would receive a minor yet simple breaking change.

**Yanked** due to a minor breaking change that we fixed in 3.10.1.

Added

* The new approach to internal schemas: instead of having them duplicate certain fields from `latest`, we introduced a new `HEAD` version -- the only one the user maintains by hand. All requests get migrated to `HEAD` and latest schemas are generated from `HEAD`. `cadwyn.structure.HeadVersion` was added to give us the ability to have migrations between `HEAD` and latest, thus eliminating the need for `InternalRepresentationOf` because all the used schemas are now the internal representations

Changed

* `latest` is now named `head` because it no longer represents the newest version. Instead, it is the the `internally used` version and the version that is used for generating all other versions.
* the newest version is not aliased from `latest` anymore. Instead, it is generated like all the rest
* deprecated `InternalRepresentationOf` and the concept of `internal schemas` in favor of `HeadVersion` migrations