Use NonNull<u8> in MmapRegionBuilder for saving space

[FuuuOverclocking]

Summary of the PR

Updates the raw_ptr field in MmapRegionBuilder (mmap_unix.rs) from Option<*mut u8> to Option<NonNull<u8>>. This modification saves 8 bytes on x86-64 systems, for example.

The public API and functionality remain unchanged.

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• All added/changed functionality has a corresponding unit/integration
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• Any newly added unsafe code is properly documented.

[FuuuOverclocking]

This commit caused the test to fail due to fn test_mmap_region_build_raw() (vm_memory::mmap_unix::tests) as addr is 0 in this test.

I'm wondering, is addr being 0 a valid parameter? Since the addr here is an HVA, it seems inappropriate to set it to 0. Instead, it could be set to, for example, 40960.

[andreeaflorescu]

@FuuuOverclocking what is the reason for this PR? 8 bytes in the great scheme of things does not feel like a lot.

[FuuuOverclocking]

In Rust, using Option<NonNull<u8>> is a common pattern when both Option and pointer are needed.

[epilys]

@FuuuOverclocking Hello, I think this PR should also wrap MmapRegion::addr in NonNull<_> as well. Would you be interested in rebasing and making this change?

[roypat]

This seems reasonable to me, I don't think there's any use case in mapping in the zero page, and Option<NonNull<u8>> is indeed the more rust-y approach.

I also agree with @epilys that if we do this, we should probably be consistent with it, and use NonNull<u8> everywhere (but in this case, this probably will need a changelog entry).

Also, it currently seems like unittests are failing, could you look into that? :o