Stop ignoring Cargo.lock

[DJMcNab]

This follows the guidance in https://blog.rust-lang.org/2023/08/29/committing-lockfiles.html

Previous discussion in #256

I've opened this as a draft, because it looks like it requires more CI work to ensure this is updated regularly.
I don't know that we want to commit to developing that

[waywardmonkeys]

I don't feel super strongly about this because I can (and have) argued both sides.

I suspect that if this does happen (or even if it doesn't), we should add CI for "minimal versions".

[DJMcNab]

To explain the history, I started making this change, then read the linked document chain, and realised it required some more care, and the advantages might not match the amount of effort needed

[xStrom]

I've been doing some research into it and have a lot more to say about this (and MSRV) and will post a comprehensive explanation in not too distant of a future, hopefully before the end of the month. Until then the short of it is that it's very messy and complicated.

We currently have Cargo.lock committed for Xilem, so in that sense not a big issue to add it here for now. Once I post my findings, hopefully we can settle on a unified strategy across repos.

[xStrom]

I did get to posting my comprehensive explanation. It is available for reading at rfcs#5.

[DJMcNab]

It seems like we'll have a more standardised experience, so I'll close this PR