Auto merge of #100298 - BlackHoleFox:hashmap_keygen_cleanup, r=Mark-Simulacrum

Replace pointer casting in hashmap_random_keys with safe code

The old code was unnecessarily unsafe and relied on the layout of tuples always being the same as an array of the same size (which might be bad with `-Z randomize-layout`)?

The replacement has [identical codegen](https://rust.godbolt.org/z/qxsvdb8nx), so it seems like a reasonable change.

Author: bors