Update all Rust dependencies to latest

[stumpylog]

As per the title, updates all the Rust dependencies to the latest versions and adds a Dependabot configuration for a monthly check on updating those.

Also updated the actions while I was at it.

Heads up that actions-rs/toolchain@v1 is now archived, and hadn't seen an update in 3 years anyway. https://github.com/dtolnay/rust-toolchain appears to be the general replacement, but it's not a drop in, so I didn't do this.