Stabilize multi-part form support #1128
Description
The multi-part support was landed in #1102
The multipart API is currently provided under unversioned, because we would like feedback on how well it works before stabilizing it. One decision we are uncertain about is that it has a lifetime parameter on the [Form] struct. That lifetime allows us to not take ownership of stuff like &[u8], &str, &mut dyn Read, etc meaning we can keep the cloning and memory allocation to a minimum. The flip side is that it's not easy to pass Form around to other threads, mpsc channels, etc.
Use this issue about feedback on the API design. For bugs, raise new issues.