Rollup merge of #68770 - ssomers:btree_drain_filter, r=Amanieu

BTreeMap/BTreeSet: implement drain_filter

Provide an implementation of drain_filter for BTreeMap and BTreeSet. Should be optimal when the predicate picks only elements in leaf nodes with at least MIN_LEN remaining elements, which is a common case, at least when draining only a fraction of the map/set, and also when the predicate picks elements stored in internal nodes where the right subtree can easily let go of a replacement element.

The first commit adds benchmarks with an external, naive implementation. to compare how much this claimed optimality-in-some-cases is actually worth.

diff --cc src/liballoc/tests/btree/map.rs
Simple merge