multiboot2-header crate - initial release

[phip1611]

After I prepared everything, the multiboot2-header crate is ready for the initial release. I've put a lot of work into it and I am confident that the code base is solid. There are some tests and I also used external tools to compare the results, for example how bootinfo parses the mb2 header.

Any additional comments @IsaacWoods ?

Small example/demo:

use multiboot2_header::builder::Multiboot2HeaderBuilder;
use multiboot2_header::{
    Multiboot2Header, HeaderTagFlag, HeaderTagISA, InformationRequestHeaderTagBuilder, MbiTagType,
    RelocatableHeaderTag, RelocatableHeaderTagPreference,
};

/// Small example that creates a Multiboot2 header and parses it afterwards.
fn main() {
    // We create a Multiboot2 header during runtime here. A practical example is that your
    // program gets the header from a file and parses it afterwards.
    let mb2_hdr_bytes = Multiboot2HeaderBuilder::new(HeaderTagISA::I386)
        .relocatable_tag(RelocatableHeaderTag::new(
            HeaderTagFlag::Required,
            0x1337,
            0xdeadbeef,
            4096,
            RelocatableHeaderTagPreference::None,
        ))
        .information_request_tag(
            InformationRequestHeaderTagBuilder::new(HeaderTagFlag::Required)
                .add_irs(&[MbiTagType::Cmdline, MbiTagType::BootLoaderName]),
        )
        .build();

    // Cast bytes in vector to Multiboot2 information structure
    let mb2_hdr = unsafe { Multiboot2Header::from_addr(mb2_hdr_bytes.as_ptr() as usize) };
    println!("{:#?}", mb2_hdr);
}

produces

Multiboot2Header {
    header_magic: 3897708758,
    arch: I386,
    length: 64,
    checksum: 397258474,
    tags: [
        InformationRequestHeaderTag {
            type: InformationRequest,
            flags: Required,
            size: 16,
            requests: [
                Cmdline,
                BootLoaderName,
            ],
        },
        RelocatableHeaderTag {
            type: Relocatable,
            flags: Required,
            size: 24,
            min_addr: 0x0000000000001337,
            max_addr: 0x00000000deadbeef,
            align: 4096,
            preference: None,
        },
        EndHeaderTag {
            typ: End,
            flags: Required,
            size: 8,
        },
    ],
}

The generated header can also be validated by crates such as bootinfo.

[IsaacWoods]

I'm afraid I won't have time to do a review of this crate at the moment, as I'm very busy what with uni starting up again in the UK atm. I've had a brief flick through at the examples and code, however, and this seems very reasonable. Feel free to merge when you're happy with it!

[phip1611]

oops, this was committed by accident. But this shouldn't change everything, because dereferencing a u8 or reading it, result both in a copy.