Auto merge of #81094 - ssomers:btree_drainy_refactor_3, r=Mark-Simulacrum

BTreeMap: split up range_search into two stages

`range_search` expects the caller to pass the same root twice and starts searching a node for both bounds of a range. It's not very clear that in the early iterations, it searches twice in the same node. This PR splits that search up in an initial `find_leaf_edges_spanning_range` that postpones aliasing until the last second, and a second phase for continuing the search for the range in the each subtree independently (`find_lower_bound_edge` & `find_upper_bound_edge`), which greatly helps for use in #81075. It also moves those functions over to the search module.

r? `@Mark-Simulacrum`

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