- `slice::sort` -> driftsort
  https://github.com/Voultapher/sort-research-rs/blob/main/writeup/driftsort_introduction/text.md

- `slice::sort_unstable` -> ipnsort
  https://github.com/Voultapher/sort-research-rs/blob/main/writeup/ipnsort_introduction/text.md

Replaces the sort implementations with tailor made ones that strike a
balance of run-time, compile-time and binary-size, yielding run-time and
compile-time improvements. Regressing binary-size for `slice::sort`
while improving it for `slice::sort_unstable`. All while upholding the
existing soft and hard safety guarantees, and even extending the soft
guarantees, detecting strict weak ordering violations with a high chance
and reporting it to users via a panic.

In addition the implementation of `select_nth_unstable` is also adapted
as it uses `slice::sort_unstable` internals.

Also includes small doc fixes.

const_trait in conjunction with specialization was
deemed not ready for usage in this scenario. So
instead a two-stage trait specialization approach
is used. This approach is likely worse for
compile-times. Future work that enables
const_trait can revert back to the previous
version as outlined in the comment marked
FIXME(effects).

Freeze + Copy types should be allowed to choose between all three
small-sort variants. With the recent changes to small-sort selection,
a regression was added that only let such types choose between network
and fallback. It can now also choose general where appropriate.

The changes made only a limited improvement for the current small
miri coverage and in general test coverage of the sort implementations.
But they exploded test times from ~13s to ~240s, which is not deemed
worth it.

Due to refactoring the const_trait usage, the CopyMarker impl was
accidentally deleted, which had the consequence that the Copy
specialization for the small-sort was never picked.

The previous message omits which of the dozens of tools called tidy is
meant. And it's written in a way that one can easily miss the *not*,
thinking it reads "Note that `tidy` is the in-tree `src/tools/tidy` but
needs to be installed". The error message should hopefully help future
contributors.