rust: Add Cargo.lock

We've been seeing more breakage recently due to much of the Rust
ecosystem sticking to minimum rustc versions more recent than ours. A
lockfile now means we have to periodically update it, but at least we
have much better control over our CI system and when we want to spend
time resolving package bump conflicts vs. just trying to get a small
patch in.

Closes: #1719
Approved by: cgwalters