Rollup merge of rust-lang#148487 - Qelxiros:148441-option-into-flat-iter, r=scottmcm

add Option::into_flat_iter

Tracking issue: rust-lang#148441

I only implemented `into_flat_iter` in this PR, but I'd be happy to add `flat_iter` / `flat_iter_mut` (equivalent to calling `as_ref` / `as_mut` first) if those are desired. See rust-lang/libs-team#626 for context.

Author: matthiaskrgr