Fix a handful of clippy lints

[smklein]

This is a collection of (hopefully) not too controversial fixes from Clippy.

Full caveat: Haven't tested this on real hardware, because I don't have any (yet!). I'd be interested in recommendations / docs on what to buy / from where.

There are some remaining "default" clippy lints I haven't yet patched:

• https://rust-lang.github.io/rust-clippy/master/index.html#missing_safety_doc appears in a variety of places; I don't have the context to be able to add these docs yet.
• https://rust-lang.github.io/rust-clippy/master/index.html#identity_op - many constants appear as "N + 0" or some equivalent, but this seems pretty intentional.
• https://rust-lang.github.io/rust-clippy/master/index.html#too_many_arguments - this only appears a couple of times, but seemed a little more invasive, so I didn't include any relevant changes here.

These lints have been explicitly allowed when executing clippy: https://github.com/oxidecomputer/hubris/pull/128/files#diff-3989253bf9ad53a1e59393d6504c2f8c9aaa7ca7b1a30ac713f83817463b6f91R44-R50

[steveklabnik]

Thanks for this! A few questions/comments.

[smklein]

Gonna let this one go - seems out-of-date enough to close