Auto merge of #77244 - ssomers:btree_love_the_leaf_edge, r=Mark-Simulacrum
Auto merge of #77244 - ssomers:btree_love_the_leaf_edge, r=Mark-Simulacrum

BTreeMap: more refactoring around edges

Continuation of #77005.

r? `@Mark-Simulacrum`

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