refactor: make CService and CNetAddr constexpr (when in cpp20)

[PastaPastaPasta]

Issue being fixed or feature implemented

Underlying motivation is making a lot of other potential stuff constexpr; CService and CNetAddr is used by quite a few things, and making it fully stack allocated (and constexpr) can be quite valuable I think

Also, I think I would like to PR this to bitcoin, so please provide review as if you were a bitcoin core developer and had very high standards :D

What was done?

In c++20, CService and CNetAddr will always be constexpr, in c++17 some of it's constructors are constexpr; removes use of prevector, so we know that CService and CNetAddr will be fully on the stack in both 17 and 20; however certain constructors that need std::copy or reinterpret_cast cannot be c++17 constexpr

How Has This Been Tested?

Building with c++17 and c++20 and ensuring that I was able to connect to ipv4 v6 and tor nodes

Breaking Changes

Should be none; this is mostly for conceptual review (and running it); I'm not positive this is completely bug free; I want to test a bit more.

Checklist:

Go over all the following points, and put an x in all the boxes that apply.

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation
• I have assigned this pull request to a milestone (for repository code-owners and collaborators only)

[github-actions]

This pull request has conflicts, please rebase.