A debug allocator which removes overalignment from align < 8 allocations

[saethlin]

This reorganizes the implementation of the System allocator to permit adding various debuggig features. Currently, all that this implements is a scheme that allocates a little extra space for low-alignment allocations then returns a pointer into the actual allocation which is offset so that it is not over-aligned.

This is a huge aid in discovering accidental reliance on over-alignment. Allocators designed for C can be relied upon to produce over-aligned pointers, so alignment-related bugs can be latent for a long time. Currently I am aware of ~100 crates in the wild where Miri detects misaligned pointer access. Many of these take a &[u8], sometimes from a Vec<u8> and attempt to do reads by converting to a more-aligned type without checking if the access is aligned.

On its own, this PR does basically nothing to detect bugs, but we already have debug assertions in a number of standard library APIs which are enabled along with this allocator in -Zbuild-std. And if I ever finish #98112, we will be able to catch uses which just do misaligned pointer dereferences directly.

This implementation is factored so accommodate other patches to the default allocator which can help in detecting other sources of UB.

@rustbot label +T-libs

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This technique I'm trying to use breaks a UI test and I do not know why and I can't figure out how to fix it. I feel like this suggestion that's now coming out of a UI test doesn't even make sense. I made the per-system System allocators pub(crate) but they're suggested outside of std I think? Help? resolved by #99091 ❤️