Merge bitcoin/bitcoin#22051: Basic Taproot derivation support for descriptors

2667366aaa69447a9de4d819669d254a5ebd4d4b tests: check derivation of P2TR (Pieter Wuille)
7cedafc5412857404e9a6c3450b100cb8ee4081a Add tr() descriptor (derivation only, no signing) (Pieter Wuille)
90fcac365e1616779b40a69736428435df75fdf2 Add TaprootBuilder class (Pieter Wuille)
5f6cc8daa83700d1c949d968a5cf0d935be337b7 Add XOnlyPubKey::CreateTapTweak (Pieter Wuille)
2fbfb1becb3c0c109cd7c30b245b51da22039932 Make consensus checking of tweaks in pubkey.* Taproot-specific (Pieter Wuille)
a4bf84039c00b196b87f969acf6369d72c56ab46 Separate WitnessV1Taproot variant in CTxDestination (Pieter Wuille)
41839bdb89b3777ece2318877b9c7921ecca2472 Avoid dependence on CTxDestination index order (Pieter Wuille)
31df02a07091dbd5e0b315c8e5695e808f3a5505 Change Solver() output for WITNESS_V1_TAPROOT (Pieter Wuille)
4b1cc08f9f94a1e6e1ecba6b97f99b73fb513872 Make XOnlyPubKey act like byte container (Pieter Wuille)

Pull request description:

  This is a subset of #21365, to aide review.

  This adds support `tr(KEY)` or `tr(KEY,SCRIPT)` or `tr(KEY,{{S1,{{S2,S3},...}},...})` descriptors, describing Taproot outputs with specified internal key, and optionally any number of scripts, in nested groups of 2 inside `{`/`}` if there are more than one. While it permits importing `tr(KEY)`, anything beyond that is just laying foundations for more features later.

  Missing:
  * Signing support (see #21365)
  * Support for more interesting scripts inside the tree (only `pk(KEY)` is supported for now). In particular, a multisig policy based on the new `OP_CHECKSIGADD` opcode would be very useful.
  * Inferring `tr()` descriptors from outputs (given sufficient information).
  * `getaddressinfo` support.
  * MuSig support. Standardizing that is still an ongoing effort, and is generally kind of useless without corresponding PSBT support.
  * Convenient ways of constructing descriptors without spendable internal key (especially ones that arent't trivially recognizable as such).

ACKs for top commit:
  Sjors:
    utACK 2667366 (based on https://github.com/bitcoin/bitcoin/pull/21365#issuecomment-846945215 review, plus the new functional test)
  achow101:
    Code Review ACK 2667366aaa69447a9de4d819669d254a5ebd4d4b
  lsilva01:
    Tested ACK https://github.com/bitcoin/bitcoin/pull/22051/commits/2667366aaa69447a9de4d819669d254a5ebd4d4b
  meshcollider:
    utACK 2667366aaa69447a9de4d819669d254a5ebd4d4b

Tree-SHA512: 61046fef22c561228338cb178422f0b782ef6587ec8208d3ce2bd07afcff29a664b54b35c6b01226eb70b6540b43f6dd245043d09aa6cb6db1381b6042667e75

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