Replace AlignedBoxedSlice with aligned_vec::ABox

[FreezyLemon]

Closes #3370.

ABox<[T]> generally works the same way as AlignedBoxedSlice<T>, but uses a different alignment by default:

AlignedBoxedSlice: 8 on wasm, 64 everywhere else ABox: Generally two cachelines, 64 bytes for unknown archs (exact rules)

I am pretty sure the wasm allocation bug mentioned in the code comment has been fixed, so using a higher alignment should be fine on the wasm32 arch.

It's possible to configure aligned_vec to reproduce the old behaviour exactly, but I think it's probably better to stick to the crate defaults.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.69%. Comparing base (3c3a26f) to head (66e15b9). Report is 13 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3371 +/- ## ========================================== + Coverage 88.52% 88.69% +0.16% ========================================== Files 89 89 Lines 28323 27760 -563 ========================================== - Hits 25074 24622 -452 + Misses 3249 3138 -111 ```

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