Constifying large traits in libstd (i.e. Iterator)

[compiler-errors]

• Allow const iterator implementations rust#102225
• implement const iterator using rustc_do_not_const_check rust#106541
• Remove do_not_const_check from Iterator methods rust#132368

My understanding is that constifying iterator currently required constifying all of its extension methods. That was a lot of work.

Instead, we just slapped rustc_do_not_const_check onto all the methods, somewhat leaving us vulnerable to CTFE errors "post-mono" if any of the combinators ended up doing anything non-const.

Instead of doing this, we probably could introduce something like rustc_non_const_trait_method that forces a specific method to be non-const in a const trait.

This means we don't need to work towards constifying all of the combinators all at once. Then when we constify iterator, we can initially slap #[rustc_non_const_trait_method] on every combinator, and slowly work to peel them off until the trait is fully constified.