ELF refactoring with breaking changes

[xoofx]

Following the lessons learned on supporting PE files, this PR is refactoring the ELF loader to make it more solid and to support full binary roundtrip with ELF binaries.

This is a breaking change, so it will require a bump to 2.0

List of tasks to finish before merging:

• Update the tests to use Verify instead of relying on readelf
• Add check for ELF binary roundtrip equality
• Re-add similar methods ElfFile.AddSection (or ElfFile.AddContent maybe, let's see)
• Check memory usage and see what can be improved (some stuffs are now using class instead of struct, but it might be better to stick to struct)
• Update documentation