zeroize 1.8.2 -> 1.9.0 performance regression

1.8.2:

```
105,300 ( 4.85%)  ???:<rustls::crypto::hmac::Tag as core::ops::drop::Drop>::drop [/tmp/.tmpcy4z8i/target/release/rustls-ci-bench]
```

1.9.0:

```
523,260 (19.26%)  ???:<rustls::crypto::hmac::Tag as core::ops::drop::Drop>::drop [/tmp/.tmppiwQse/target/release/rustls-ci-bench]
```

This function calls `[u8; 64].zeroize()` in both cases.