`RUSTC_ALLOW_UNSTABLE_<feature>`: a `RUSTC_BOOTSTRAP` alternative

[jyn514]

This RFC was in large part drafted in May 2025, shortly after my Rust All Hands talk, "Futile Feature Gates". There were about 25 people in that room; I want to thank all of them for their feedback during that session, and everyone who reviewed early drafts of this RFC.

Rendered

[!IMPORTANT]
When responding to RFCs, try to use inline review comments (it is possible to leave an inline review comment for the entire file at the top) instead of direct comments for normal comments and keep normal comments for procedural matters like starting FCPs.

This keeps the discussion more organized.