builder: use new abstraction to guarantee alignment

[phip1611]

Before this PR, both builder structs (HeaderBuilder, BootInformationBuilder) build their corresponding structures in a Vec<u8>. However, there is no guarantee of an 8-byte alignment in that case. As long as the allocator_api is not stable, I chose the following workaround:

Use a regular Vec<u8> with a capacity of expected_len + 7. This way, I can add zeroes at the beginning to the vec for alignment adjustments. A wrapper type returns a slice to the actual and aligned bytes of the corresponding structure, i.e, the wrapper type knows an offset between 0 and 7 where the structure actually starts.

The only problem with this type: I can't create a reliable unit test for the alignment offsetting magic. I created a local test run using the allocator_api and a "weird" (unaligned) pointer from the allocator and everything worked as expected. I tested the new code with miri and miri reported no allocation or alignment errors 🥳

[phip1611]

@YtvwlD I'm curious. Did you encountered problems with the builder in your project due to alignment issues?

If the allocator "is friendly", you might never have encountered any problems. Unfortunately, this is very hard to unit-test without the allocator-api feature.

[phip1611]

Ah, interesting. On Windows, the allocator triggered one of my new assertions

https://github.com/rust-osdev/multiboot2/actions/runs/5344793550/jobs/9689626355?pr=156

[phip1611]

Strictly speaking, the Vec -> Box conversion is overkill. I could also keep the vector in the byte wrapper.

[YtvwlD]

I did not encounter any errors yet. But you're right: According to the spec the struct should be 8-byte aligned.