Update alloc, core, and std to rand 0.8

[notriddle]

No description provided.

[rustbot]

r? @Mark-Simulacrum

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[rustbot]

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If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

• Stabilizing library features
• Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
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[klensy]

see #104658

[Mark-Simulacrum]

r? @thomcc

[thomcc]

This will fail on wasm32-unknown-unknown. The right way to do this is #104658.

[thomcc]

See previous attempts to update this:

• Update rand deps #86963 (in particular Update rand deps #86963 (comment) which explains why this is non-trivial)
• Update rand to 0.8 in library/ and compiler/ #89131
• Avoid using rand::thread_rng in the stdlib benchmarks. #96626 (comment) (I tried in that PR at the same time, but settled for just removing the usage of thread_rng() from the benchmarks, since that was the main goal).
• Upgrade rand to v0.8 in libstd #104185