Add std::unordered_set support and a helper BuildList to dedup list build handlers #277

Add std::unordered_set support and a helper BuildList to dedup list build handlers that is being used for map, set and vector.

While looking bitcoin/bitcoin#29409, found a TODO noting that libmultiprocess lacked std::unordered_set support, requiring downstream that PR to implement the build/read functions locally.

I believe there could be more dedup adding a ReadList too. I could do that as a follow-up if desirable

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #276 (build: prepare for subtree split by Sjors)
• #243 (mpgen: support primitive std::optional struct fields by ryanofsky)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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