Architecture Specific Relocations + 32bit Support

[landhb]

@gz Here is the full PR for #29. It ended up being larger than I originally planned so if you prefer I try to split it up into multiple just let me know.

Adds RelocationEntry struct to abstract 32bit and 64bit Rela<T>/Rel<T> entries.

pub struct RelocationEntry {
    pub rtype: RelocationType,
    pub offset: u64,
    pub index: u32,
    pub addend: Option<u64>,
}

Which is then passed to the user via the relocate() function in the exposed trait interface:

pub trait ElfLoader {
    ...
    /// Request for the client to relocate the given `entry`
    /// within the loaded ELF file.
    fn relocate(&mut self, entry: RelocationEntry) -> Result<(), ElfLoaderErr>;
    ...
}   

Architecture specific relocation types are added into ./arch/[ARCH].rs and can be handled in a single match statement by the user by adding a use for the architectures they'd like to handle:

use elfloader::arch::x86_64::RelocationTypes::*;
use elfloader::arch::x86::RelocationTypes::*;
use elfloader::RelocationType::{x86, x86_64};

// Handle relocations for multiple architectures
fn relocate(&mut self, entry: RelocationEntry) -> Result<(), ElfLoaderErr> {
    match entry.rtype {

        // Example handling x86 relocation type
        x86(R_386_RELATIVE) => {},

        // Example handling x86_64 relocation type
        x86_64(R_AMD64_RELATIVE) => {},

        // Unsupported Skip
        x => {trace!("Skipping relocation type: {:?}", x)}
    }
    Ok(())
}

Change summaries:

• Added ElfBinary::get_arch()
• Added ElfBinary::interpreter()
• Added tests load_pie_elf_32(), check_nopie_32(), and check_tls_32()
• Edited ElBinary::maybe_relocate() to handle SectionData::Rela32, SectionData::Rel32, and SectionData::Rel64 and pass the abstracted RelocationEntry object to loader.relocate()
• Edited ElfBinary::parse_dynamic() to handle 32bit SegmentData::Dynamic32
• Edited ElfBinary::load() to handle 32bit ProgramHeader::Ph32
• Edited ElfBinary::iter_loadable_headers() to handle 32bit ProgramHeader::Ph32
• Edited ElfBinary::new() to handle 32bit ProgramHeader::Ph32
• Edited README to reflect usage

Repo structure changes:

• Moved test code to src/test.rs
• Moved ElfBinary code to src/binary.rs
• Added src/arch/* for architecture specific Relocation definitions

[gz]

This looks awesome, thanks for the contribution! I will review this over the next few days when I have some time.

[landhb]

No problem!

[gz]

release as 0.15, many thanks for the great contribution!

[landhb]

Thanks!