Implement Supertrait auto impl #636

Summary

Within the 2026 goal period we strive for completion of the following items.

• Implementation of the core language features stipulated by the [RFC #3851].
• Continuous update on the RFC for errata to reflect necessary changes as implementation moves along.
• Resolve the impl overlapping question, possibly as an optional feature behind an associated feature gate.
• Field trial of the standard library trait refactoring with the supertrait_auto_impl feature gate.
• Implementation of the optional features and lints spelled in the RFC as much as possible.

This issue is intended for status updates only.

For general questions or comments, please contact a goal owner or post in the Zulip topic.