Many AsciiExt imports have become useless thanks to the inherent ascii
methods added in the last commits. These were removed. In some places, I
fully specified the ascii method being called to enforce usage of the
AsciiExt trait. Note that some imports are not removed but tagged with
a `#[cfg(stage0)]` attribute. This is necessary, because certain ascii
methods are not yet available in stage0. All those imports will be
removed later.

Additionally, failing tests were fixed. The test suite should exit
successfully now.

This includes the following stabilizations:

- tcpstream_connect_timeout https://github.com/rust-lang/rust/pull/44563
- iterator_for_each https://github.com/rust-lang/rust/pull/44567
- ord_max_min https://github.com/rust-lang/rust/pull/44593
- compiler_fences https://github.com/rust-lang/rust/pull/44595
- needs_drop https://github.com/rust-lang/rust/pull/44639
- vec_splice https://github.com/rust-lang/rust/pull/44640

Like #43008 (f6689991), but _much more aggressive_.

When trying to use dedup_by to merge some auxiliary information from
removed elements into kept elements, I was surprised to observe that
vec.dedup_by(same_bucket) calls same_bucket(a, b) where b appears
before a in the vector, and discards a when true is returned.  This
argument order is probably a bug, but since it has already been
stabilized, I guess we should document it as a feature and move on.

(Vec::dedup also uses == with this unexpected argument order, but I
figure that’s not important since == is expected to be symmetric with
no side effects.)
Signed-off-by: NAnders Kaseorg <andersk@mit.edu>

This reverts commit 143206d5.

Move it and Bound to core::ops while we're at it.

Closes #30877

Replaced by adding extra imports, adding hidden code (`# ...`), modifying
examples to be runnable (sorry Homura), specifying non-Rust code, and
converting to should_panic, no_run, or compile_fail.

Remaining "```ignore"s received an explanation why they are being ignored.

These were found by running tidy on stable versions of rust and finding
features stabilised with the wrong version numbers.

Fixes #37746

Fixes #37866

Fixes #38723.