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

As proposed by @bitwalker in [this comment](https://github.com/0xMiden/compiler/pull/1023#pullrequestreview-4001973009):

> [...] 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.

The issue depends on #1028 for section size breakdowns. Start by reviewing that PR to understand the package structure and size reporting. The new `diff` subcommand will need to be added to `objtool`, likely in a file like `src/objtool/diff.rs`. It should compare two `.masp` files, outputting size differences per section. Check existing `objtool` commands for patterns on argument parsing and output formatting.

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