Rollup merge of #70850 - ssomers:btreemap_first_last, r=Amanieu
Rollup merge of #70850 - ssomers:btreemap_first_last, r=Amanieu

BTreeMap first last proposal tweaks

Clean-up and following up on a request in #62924.

Trying the reviewer of the original code #65637...
r? @scottmcm

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