Cargo.lock in repository

[jaybosamiya]

At some point, we should consider maintaining a Cargo.lock in the repository.

It was originally .gitignored in 2021: d4ded8f

#360 tried to (re)introduce it (including some rationale for why we should be including Cargo.lock). Since non-trivial changes were soon to be made (to move off the rustc fork), adding Cargo.lock right now as decided against.

Likely, once we have moved off of the rustc fork, we should revisit this and lock our dependencies to make longer term maintenance easier.

If any more reasons (other than the rustc fork move) exist to not keep the Cargo.lock around, it would be good to hear them, thus this discussion thread. For reasons to maintain Cargo.lock, some are already listed on #360.

[utaal]

Being added by @jaybosamiya in #669.