bootspec: offer TryInto conversion from Generation to any schema version

[cole-h]

This utilizes the derive_more crate so that we don't have to manually add new implementations for each version.

Description

Closes #109.

Checklist

• Built with cargo build
• Formatted with cargo fmt
• Linted with cargo clippy
• Ran tests with cargo test
• Added or updated relevant tests (leave unchecked if not applicable)
• Added or updated relevant documentation (leave unchecked if not applicable)

[cole-h]

While this current implementation is infallible (because we don't have any other variants), it will be come fallible as soon as we add a new variant.

[cole-h]

I just realized that "any" here is kinda incorrect... Since deserializing the BootSpec will only return the latest generation version. So, once we have one more version, it will only ever deserialize the latest version (the first one in the enum) (if it exists)...

Maybe for v2 we should change BootSpec's generation field to be a Vec<Generation>... I think that might require a custom deserializer, but it also might not. Then we could maybe provide a helper function for "give me version X if it exists (and error otherwise)"...

[cole-h]

I think this is "not bad" for the moment, though. Assuming @RaitoBezarius doesn't report anything wrong with it, I'll probably cut a 1.0.0 tomorrow.

[RaitoBezarius]

Will give it a try in some minutes.