Merge bitcoin/bitcoin#24860: Miniscript integration follow-ups

f3a50c9dfe645c548713e44e0eaf26ea9917a379 miniscript: rename IsSane and IsSaneSubexpression to prevent misuse (Antoine Poinsot)
c5fe5163dc31db939c44129f2ff8283b290a9330 miniscript: nit: don't return after assert(false) (Antoine Poinsot)
7bbaca9d8d355a17348a8d01e3e2521c5de466b0 miniscript: explicit the threshold size computation in multi() (Antoine Poinsot)
8323e4249db50d46ae4f43c1d8a50666549ae938 miniscript: add an OpCode typedef for readability (Antoine Poinsot)
7a549c6c59e6babbae76af008433426c6fa38fe2 miniscript: mark nodes with duplicate keys as insane (Antoine Poinsot)
8c0f8bf7bc3750fad648af1a548517a272114bca fuzz: add a Miniscript target for string representation roundtripping (Antoine Poinsot)
be34d5077b2fede7404de7706362f5858c443525 fuzz: rename and improve the Miniscript Script roundtrip target (Antoine Poinsot)
7eb70f0ac0a54adabc566e2b93bbf6b2beb54a79 miniscript: tiny doc fixups (Antoine Poinsot)
5cea85f12cba5dcfe3a298eddfa711f582adffac miniscript: split ValidSatisfactions from IsSane (Antoine Poinsot)
a0f064dc1474a048e236bfff12f4def3aa11daf3 miniscript: introduce a CheckTimeLocksMix helper (Antoine Poinsot)
ed45ee3882e69266d550b56ff69388e071f0ad1b miniscript: use optional instead of bool/outarg (Antoine Poinsot)
1ab8d89fd1bdb3c0f2a506b4a10df6c23ba21c48 miniscript: make equality operator non-recursive (Antoine Poinsot)
5922c662c08a061b3b3d5ac34a31f9f9d4640d47 scripted-diff: miniscript: rename 'nodetype' variables to 'fragment' (Antoine Poinsot)
c5f65db0f03b52bc4525acae944173829290ce6f miniscript: remove a workaround for a GCC 4.8 bug (Antoine Poinsot)

Pull request description:

  The Miniscript repository and the Miniscript integration PR here have been a moving target for the past months, and some final cleanups were done there that were not included here. I initially intended to add some small followup commits to #24148 but i think there are enough of them to be worth a followup PR on its own.

  Some parts of the code did not change since it was initially written in 2019, and the code could use some modernization. (Use std::optional instead of out args, remove old compiler workarounds).
  We refactored the helpers to be more meaningful, and also did some renaming. A new fuzz target was also added and both were merged in a single file. 2 more will be added in #24149 that will be contained in this file too.

  The only behaviour change in this PR is to rule out Miniscript with duplicate keys from sane Miniscripts. In a P2WSH context, signatures can be rebounded (Miniscript does not use CODESEPARATOR) and it's reasonable to assume that reusing keys across the Script drops the malleability guarantees.
  It was previously assumed such Miniscript would never exist in the first place since a compiler should never create them. We finally agreed that if one were to exist (say, written by hand or from a buggy compiler) it would be very confusing if an imported Miniscript descriptor (after #24148) with duplicate keys was deemed sane (ie, "safe to use") by Bitcoin Core. We now check for duplicate keys in the constructor.

  This is (still) joint work with Pieter Wuille. (Actually he entirely authored the cleanups and code modernization.)

ACKs for top commit:
  sipa:
    utACK f3a50c9dfe645c548713e44e0eaf26ea9917a379 (with the caveat that a lot of it is my own code)
  sanket1729:
    code review ACK f3a50c9dfe645c548713e44e0eaf26ea9917a379. Did not review the fuzz tests.

Tree-SHA512: c043325e4936fe25e8ece4266b46119e000c6745f88cea530fed1edf01c80f03ee6f9edc83b6e9d42ca01688d184bad16bfd967c5bb8037744e726993adf3deb

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