Disable structural records by default, allow them selectively in rustc while moving to structs

[catamorphism]

I'm working on denying the structural_records option by default in lint, and adding a legacy_records attribute so we can allow them while updating code.

Also see #4257

[catamorphism]

I added the attribute and set the lint option to deny by default. I'll open a new issue for removing them from libraries.