Rollup merge of #135367 - Urgau:unreach_pub-std-3, r=Noratrieb

Enable `unreachable_pub` lint in `alloc`

This PR enables the [`unreachable_pub`](https://doc.rust-lang.org/rustc/lints/listing/allowed-by-default.html#unreachable-pub) lint as warn in the `alloc` crate.

Most of changes are in the btree implementation and in tests.

*The diff was mostly generated with `./x.py fix --stage 1 library/alloc/ -- --broken-code`, as well as manual edits for code in macros and in tests.*

Continuation of #134286 and #135366

r? libs

Author: matthiaskrgr