Rollup merge of #143111 - xu-cheng:btreeset_from_iter, r=Mark-Simulacrum

BTreeSet: remove duplicated code by reusing `from_sorted_iter`

The method `BTreeSet::from_sorted_iter` was introduced in 49ccb7519f55bd117d2ab50b7a030637f380aec6, but it was not consistently used throughout the codebase. As a result, some code redundantly reimplemented its logic. This commit fixes the problem.

Author: matthiaskrgr