Rollup merge of rust-lang#106541 - fee1-dead-contrib:no-const-check-no, r=thomcc

implement const iterator using `rustc_do_not_const_check`

Previous experiment: rust-lang#102225.

Explanation: rather than making all default methods work under `const` all at once, this uses `rustc_do_not_const_check` as a workaround to "trick" the compiler to not run any checks on those other default methods. Any const implementations are only required to implement the `next` method. Any actual calls to the trait methods other than `next` will either error in compile time (at CTFE runs), or run the methods correctly if they do not have any non-const operations. This is extremely easy to maintain, remove, or improve.

Author: Dylan-DPC