Added elf library for elf code.

[an-owl]

I needed to get the full ELF header I figured I'd implement #247 in the process.

• In the front end everything is basically the same except I've added a method to get the whole section header.
• In the backend I've removed all the magic numbers and replaced them with constants from ::elf::abi
• I've added a helper for the section header pointer, it handles offsetting and fetching the section header, it uses an intermediate type so callers can eliminate separate code paths.
• I removed the "strlen" code in favour of using core::ffi::CStr to handle it. Originally I wanted to use CStr to store the section name in ElfSection however it doesn't implement Copy
• ElfSection::string_table never needed to be unsafe, it is no longer unsafe
• ELF defines a program/user defined block of section header values which I've added.
• The ElfSectionInner trait is no longer needed because it now uses elf::section::SectionHeader for both ELF32 and ELF64. I've left it in solely for familiarity of others. I can remove it if you want.
• ElfSection now copies the section header instead of maintaining a pointer to it. I can revert this behavior if you want.

[phip1611]

awesome, thanks! I don't think I have to look into this in the next week, but let's see.