Core features cleanup

[m-ou-se]

This sorts and categorizes the #![features] in core and removes unused ones.

This is part of #87766

The following feature attributes were unnecessary and are removed:

// Library features:
-#![feature(bool_to_option)]
-#![feature(char_indices_offset)]
-#![feature(pin_deref_mut)]
-#![feature(str_split_as_str)]
-#![feature(str_split_inclusive_as_str)]

// Language features:
-#![feature(arbitrary_self_types)]
-#![feature(custom_inner_attributes)]
-#![feature(nll)]

[rust-highfive]

r? @dtolnay

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[dtolnay]

Nice!

[dtolnay]

@bors r+

[bors]

📌 Commit 7306c69 has been approved by dtolnay

[bors]

☔ The latest upstream changes (presumably #86155) made this pull request unmergeable. Please resolve the merge conflicts.

[jyn514]

@bors rollup=iffy

[m-ou-se]

Rebased after another PR already removed unwind_attributes attriute.

@bors r=dtolnay

[bors]

📌 Commit cdf83c0 has been approved by dtolnay

[bors]

⌛ Testing commit cdf83c0 with merge 5caae6edbc5c6a605e585a5b30d5eab6c325c427...

[bors]

💔 Test failed - checks-actions

[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain)

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[jyn514]

dist-mingw failed without logs.

@bors retry

[bors]

⌛ Testing commit cdf83c0 with merge 2f07ae4...

[bors]

☀️ Test successful - checks-actions
Approved by: dtolnay
Pushing 2f07ae4 to master...