Rollup merge of #127446 - zachs18:miri-stdlib-leaks-core-alloc, r=Mark-Simulacrum

Remove memory leaks in doctests in `core`, `alloc`, and `std`

cc `@RalfJung`  https://github.com/rust-lang/rust/issues/126067 https://github.com/rust-lang/miri/issues/3670

Should be no actual *documentation* changes[^1], all added/modified lines in the doctests are hidden with `#`,

This PR splits the existing memory leaks in doctests in `core`, `alloc`, and `std` into two general categories:

1. "Non-focused" memory leaks that are incidental to the thing being documented, and/or are easy to remove, i.e. they are only there because preventing the leak would make the doctest less clear and/or concise.
    - These doctests simply have a comment like `# // Prevent leaks for Miri.` above the added line that removes the memory leak.
    - [^2]Some of these would perhaps be better as part of the public documentation part of the doctest, to clarify that a memory leak can happen if it is not otherwise mentioned explicitly in the documentation  (specifically the ones in `(A)Rc::increment_strong_count(_in)`).
2. "Focused" memory leaks that are intentional and documented, and/or are possibly fragile to remove.
    - These doctests have a `# // FIXME` comment above the line that removes the memory leak, with a note that once `-Zmiri-disable-leak-check` can be applied at test granularity, these tests should be "un-unleakified" and have `-Zmiri-disable-leak-check` enabled.
    - Some of these are possibly fragile (e.g. unleaking the result of `Vec::leak`) and thus should definitely not be made part of the documentation.

This should be all of the leaks currently in `core` and `alloc`. I only found one leak in `std`, and it was in the first category (excluding the modules `@RalfJung` mentioned in https://github.com/rust-lang/rust/issues/126067 , and reducing the number of iterations of [one test](https://github.com/rust-lang/rust/blob/master/library/std/src/sync/once_lock.rs#L49-L94) from 1000 to 10)

[^1]: assuming [^2] is not added
[^2]: backlink

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