[MICRO]: lntypes: Add Dual[A] primitive type

[ProofOfKeags]

This commit introduces a new type Dual[A] to make it easier to manage symmetric configurations or state for lightning channels.

Change Description

I'm extracting this commit from #8755 because it is proving to be useful in #8270 as well. It has applications anywhere we have symmetric structure, which in the channel state machine is ubiquitous.

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[carlaKC]

Drive by request to include d8b695a -> 77586e2 from #8270 to this PR?

[ProofOfKeags]

Drive by request to include d8b695a -> 77586e2 from #8270 to this PR?

I think those commits are better suited in the PR they are in as they are primarily motivated by the consolidation of the NumPendingUpdates function. Whenever we introduce these new fundamental structures there is a whole continuum of things that could be refactored to use them and I've been advised that incremental is the right approach. Since those specific refactors aren't just for their own sake but rather to set the stage for cae5f36, I do not believe they should be split up. Of course that commit is further motivated by the quiescer's desire to know whether it can send/receive an stfu, so the motivations cascade.

This specific commit actually did not originate in #8270 but instead from #8755 (94ccf88). I carved it off later because I realized it can be used in the #8270 work as well. All this is to say, I think it is properly scoped as is.

[carlaKC]

Since those specific refactors aren't just for their own sake but rather to set the stage for cae5f36, I do not believe they should be split up.

While I'm wary of getting off the refactor tractor and never getting off, I think that pre-factor PRs are well justified when:

1. The refactor is a reasonable unit of improvement on its own
2. Separating it helps focus review on the core change of a larger PR/series

I think those commits are better suited in the PR they are in as they are primarily motivated by the consolidation of the NumPendingUpdates function.

Without them this PR just introduces dead code, and doesn't really shift any review burden away from subsequent PRs. I think it's totally okay for a prep-work PR to exist in the context of its followup, especially when it's a nice cleanup like this.

[ProofOfKeags]

I don't mind this, but I've also been specific guidance by other members of the team at other times that they are less likely to review code that "doesn't seem to do anything". Packing stuff into Duals on its own doesn't appear useful to people and its utility is only realized when we start to index into them in the core systems. If I can get another member of the team to co-sign your request to include those commits in a pre-factor PR, I'll do it. What I'm wary of is making changes to get your sign off that causes me to be at direct odds with the requests of other members of the team.

@yyforyongyu @guggero what do you think? If you're on board with what @carlaKC is asking, then I'm happy to make this the plan.

I don't have a preference, I just want y'all to agree with each other so we can ship stuff plz.

[guggero]

LGTM, just two nits and a naming suggestion. But non-blocking.

[guggero]

I don't have a preference, I just want y'all to agree with each other so we can ship stuff plz.

I think this PR is fine on its own. To me the goal/context is clear, since the PR that's going to use is already up.

[carlaKC]

Still feel like we should slim down the dependent PR by adding the switchover to use Dual where we need it, but lgtm!

[Roasbeef]

LGTM 🪢