feat(objtool): diff two packages in terms of size

As proposed by @bitwalker in this comment:

[...] provide a lower-level plumbing subcommand, diff, which compares two packages directly in terms of their size and where that size comes from - but the actual responsibility for producing those packages is left up to a higher-level tool, i.e.

$ cargo miden build --debug -o a.masp
$ midenc asm/miden-project.toml -o b.masp
$ objtool diff a.masp b.masp

Depends on #1028 for breaking down sizes per sections.