Remove unused deps more aggressively

[macladson]

Proposed Changes

I was playing around with cargo machete and noticed that it more aggressively detects unused deps than cargo udeps.
After running cargo machete --with-metadata and going through the list, despite a few false positives, there were quite a few (seemingly) unused dependencies.

Additional Info

Maybe we could consider switching to cargo machete over cargo udeps (or potentially run both) although I'd be concerned with the number of false positives. More conservative settings find significantly less and so is less compelling

[michaelsproul]

Just a couple of Qs about things that look odd to me.