Adopt cargo workspace improvements in Rust 1.64

[tgeoghegan]

Rust 1.64 brings some cargo improvements for sharing values like crate version, dependencies, MSRV and others between member crates of a workspace. We should move Janus' MSRV to 1.64 and use those features, which will make version bump PRs like #583 simpler.

[tgeoghegan]

Using the shared dependencies is likely blocked on dependabot support.

[divergentdave]

#640 did this for our main branch. Dependabot support has landed in the meantime.

[branlwyd]

Is there any additional work to be done for this issue? So far we have pulled workspace declarations (crate version, license, etc) out to the workspace-level Cargo.toml, and moved a few sensible dependency declarations out too.

I think we probably won't bump the 0.1 branch to a >= 1.64 MSRV, so this issue doesn't apply to that branch.

We can move additional dependencies out as it makes sense to do so, but I think that is not worth keeping this issue open. I'm going to close this issue out, if there are important bits worthy of continued tracking, please reopen w/ an explanation.