Refactors digest-related code in src/lock.rs

[xfbs]

This moves the digest-related code out of src/lock.rs and adds some tests for it. This increases overall test coverage (line coverage) by about 9%.

Notes:

• We currently manually implement Serialize and Deserialize, this is usually not a good idea. It is better (less code) to use something like serde_with to implement it.
• There is probably still a couple low hanging fruit but this PR tests the most important stuff.

Before:

After:

[asmello]

Sorry I'm late to review, but I don't think this is good practice @xfbs. First time I've seen tests outside of a module gated by #[cfg(test)] and after reviewing the docs I'm pretty sure there's a good reason for that.