remove HRTB from `[T]::is_sorted_by{,_key}`

[lukas-code]

Changes the signature of [T]::is_sorted_by{,_key} to match [T]::binary_search_by{,_key} and make code like https://github.com/rust-lang/rust/issues/53485#issuecomment-885393452 compile.

Tracking issue: https://github.com/rust-lang/rust/issues/53485

~~Do we need an ACP for something like this?~~ Edit: Filed ACP here: https://github.com/rust-lang/libs-team/issues/121

[rust-highfive]

r? @Mark-Simulacrum

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[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

• Stabilizing library features
• Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
• Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[Mark-Simulacrum]

Yes, I think an ACP or similar would make sense. Marking as waiting on libs-api.

[m-ou-se]

@bors r+

[bors]

:pushpin: Commit f3d7b39cdf483ae543757fbef369c166650e66f2 has been approved by m-ou-se

It is now in the queue for this repository.

[lukas-code]

since this feature is still unstable @rustbot label -relnotes