Enforce buffer alignment

[mkeeter]

This is a follow-up to #1105, so only look at the last commit (b78c987). I'm marking it as a draft until #1105 is merged; then I'll rebase it onto main.

This PR makes Buffer operations stricter: reads and writes now panic if we try to access a Buffer without block alignment. To make this less dangerous, the relevant functions are changed from pub to pub(crate), so they can't be called from the public API. Propolis only uses Buffer::new and Buffer::as_vec, so that shouldn't be an issue.

Within crucible-upstairs, I had to update the implementation of IOSpan to do read-modify-write operations; previously, three unit tests would panic under the strict regime.

[mkeeter]

This is now rebased and ready for review!