Remove negate_unsigned feature gate

[oli-obk]

fixes#29645

[rust-highfive]

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[alexcrichton]

Thanks! Could you also squash the commits down into one?

[hanna-kruppe]

This will break nightly programs that use the negate_unsigned feature, right? If so, please throw a [breaking-change] in the PR description or in one of the commit messages.

[oli-obk]

squashed, added [breaking-change] and rewrote the help message

[ollie27]

I think the help message should mention .wrapping_neg() because it matches the behaviour of the current unsigned negation and can't overflow.

[pnkfelix]

@ollie27 I am not sure if wrapping_neg is a const fn, so such a message could be misleading in a const expression context. .. still there might be a way to phrase the suggestion (or only emit it in non const contexts? )

[pnkfelix]

The Travis failures look legitimate

[oli-obk]

fixed tidy errors and adjusted the second test's help messages

[emberian]

(Statement retracted, nuke it from orbit)

[pnkfelix]

@bors r+ cb0858b

[pnkfelix]

@bors rollup

[bors]

⌛ Testing commit cb0858b with merge 38683b9...

[bors]

💔 Test failed - auto-linux-64-nopt-t

[steveklabnik]

@bors: r-

this failed the rollup

[pnkfelix]

@oli-obk looks like it needs to be updated to use the new structured error API for the span_note call?

[oli-obk]

rebased and fixed

[pnkfelix]

@bors r+ bac3eec rollup