Rollup merge of #51326 - sdroege:slice-iter-cleanup, r=dtolnay
Rollup merge of #51326 - sdroege:slice-iter-cleanup, r=dtolnay

Various minor slice iterator cleanups

See individual commits

aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust