Tracking issue for RFC 2632 `?const` trait bound opt-out

[ecstatic-morse]

This is the tracking issue for the ?const opt-out for trait bounds described in rust-lang/rfcs#2632. The primary tracking issue for that RFC is #67792. The feature gate for it is const_trait_bound_opt_out.

This RFC has not yet been accepted. It is being implemented on a provisional basis to evaluate the potential fallout.

cc #57563

Implementation steps:

• Implement (Implement ?const opt-out for trait bounds #68140)
• Forbid ?const outside const fn or impl const Trait