Possible UB in min_heap

[KaiserKarel]

Running miri on storage::collections results in the following error:

test collections::binary_heap::tests::min_heap_works ... error: Undefined Behavior: trying to reborrow for SharedReadOnly at alloc1652259+0x4, but parent tag <4076562> does not have an appropriate item in the borrow stack
    --> /home/karel/.rustup/toolchains/nightly-2021-05-06-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/cmp.rs:1354:37
     |
1354 |             PartialOrd::partial_cmp(*self, *other)
     |                                     ^^^^^ trying to reborrow for SharedReadOnly at alloc1652259+0x4, but parent tag <4076562> does not have an appropriate item in the borrow stack
     |
     = help: this indicates a potential bug in the program: it performed an invalid operation, but the rules it violated are still experimental
     = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information

I'm working on #847, which requires significant unsafe to implement split_at_mut, so running Miri in CI seems appropriate too,

[HCastano]

I was under the impression we already ran Miri as part of the CI 🤔
Feel free to open a PR which adds it.

As for the potential bug, it's unclear to me what Miri is trying to tell us here. Definitely worth looking into though

[KaiserKarel]

I haven't actually looked into the cause or CI configuration, but I assumed that since Miri errored it was not run.

I'll look into enabling it in a PR, and will post the full error tomorrow