Don't panic in Vec::shrink_to_fit

[cuviper]

We can help the compiler see that Vec::shrink_to_fit will never reach the panic case in RawVec::shrink_to_fit, just by guarding the call only for cases where the capacity is strictly greater. A capacity less than the length is only possible through an unsafe call to set_len, which would break the Vec invariants, so shrink_to_fit can just ignore that.

This removes the panicking code from the examples in both #71861 and #75636.

[rust-highfive]

r? @LukasKalbertodt

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[Mark-Simulacrum]

Comparison should mostly be the same independent of != vs. > I think perf wise, so @bors r+ rollup=never

[bors]

📌 Commit 7551f3f has been approved by Mark-Simulacrum

[pickfire]

Looks good to me but today I learned that we can do test like this.

[bors]

⌛ Testing commit 7551f3f with merge c03c213...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: Mark-Simulacrum
Pushing c03c213 to master...