Add a chk package for checked types

[eljobe]

The package name is intentionally very short because it will be included in function and method signatures and the code may become unwieldy if the package name is something like constraints.

The motivation for attempting to add a package like this was born out of wanting to simplify some code in the bold repo where we were checking that virtual > 0 in way too many places. But, I think this could be generally useful and would like to have it in the nitro repository so we can start adopting it in the codebase. Alternatively, if we think it would be a generally useful library, we could pull it out into a small repository and let it stand on its own.

The main purpose of the pull request is to see if we have a collective desire to use this kind of library, and, if so, where we would like it to live.

[PlasmaPower]

I'm not sold on the UX tradeoff here, especially given that Go lacks operator overloading. My experience in Rust with the NonZero type hasn't been great in terms of the effort to create it vs safety, and the Go type seems a bit worse there in that it's hard to add two Pos64s for example. I'd like to see an example of how this would be used for a real function, and compare it there, as I'd worry it'd be better to just have a check in the function accepting a chk.Pos64 to error if the input uint64 is zero instead.

Side note -- I need to brush up on my Go inheritance. Can I do a > b for Pos64s? That's only if it's declared as type Pos64 = uint64 right? How about x := chk.Pos64 {0} ? Hopefully it's treated as a private value and that's not allowed.

[eljobe]

The point about lack of operator overloading is definitely a good one. It is the biggest tradeoff I see in taking this approach.

As to your specific questions about what's allowed:
No, you can't a > b because > is not implemented on struct.
No, you can't x := chk.Pos64{0} because it's a private value. But, you can x := chk.Pos64{} because all public types can have zero values. That's why I made this type's "zero" value 1.

I'm happy to abandon this approach. Ideally, we'd also capture an Architecture Decision Record explaining why we don't want to consider this in our codebase in the future. It's great to be able to reference decisions like this in the future (in case someone is tempted to do this later.) What do you think about the idea of trying to get in the habit of creating those kinds of records?

[PlasmaPower]

Yeah, that sounds good to me. I'm definitely up for revisiting this any time we encounter a package we think we could use this type btw. It might be that it doesn't make sense to use this type in most places, but there's a couple places in the codebase where it's helpful.