Rollup merge of #123050 - RalfJung:panic_str, r=m-ou-se

panic_str only exists for the migration to 2021 panic macros

The only caller is `expect_failed`, which is already a cold inline(never) function, so inlining into that function should be fine. (And indeed `panic_str` was `#[inline]` anyway.)

The existence of panic_str risks someone calling it when they should call `panic` instead, and I can't see a reason why this footgun should exist.

I also extended the comment in `panic` to explain why it needs a `'static` string -- I know I've wondered about this in the past and it took me quite a while to understand.

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