Tidy Cargo dependencies

[paulhauner]

Description

Cargo dependencies (Cargo.toml) could use some love.

I suggest we:

• Remove any unused dependencies (I suspect there are many).
• Add a caret (^) to deps we want to keep upgrading.
• Add Cargo.lock to git.

[michaelsproul]

Deps without any signifier are implicitly taken to be caret dependencies, so bullet (2) may not be necessary: https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#specifying-dependencies-from-cratesio

[paulhauner]

Related to #403

[jzaki]

cargo fix with unused_imports lint