Add humility-caboose and humility-cmd-caboose

[mkeeter]

It is helpful to read the caboose without knowing anything about the image (e.g. no archive). Fortunately, that is part of the caboose's design! This PR adds a humility caboose read subcommand which prints a raw caboose.

It is a library-first PR:

• humility-caboose is a library for pulling the caboose from an image, with strongly typed errors
• humility-cmd-caboose is a wrapper which prints it out (and is linked into the main humility binary)
• The library crate is fully documented (with #![warn(missing_docs)])

$ humility caboose read
humility: attached via CMSIS-DAP
humility: found flash base at 0x8000000 (pc at 0x804e574)
humility: found image header at 0x8000298
humility: found caboose at 0x8088000..0x8088100
[
    ("GITC", [
        "e0297c8596c90552f6258841d96212b9061277ab-dirty",
    ]),
    ("BORD", [
        "grapefruit-a",
    ]),
    ("NAME", [
        "grapefruit-a",
    ]),
]

There's a bit of copy-pasta from hubtools, but that implementation expects to operate on a HubrisArchive in memory (rather than a raw byte array). I think the copy-pasta is minimal / stable enough to not be a big deal, but can revisit if folks feel strongly about it.