Merge #14646: Add expansion cache functions to descriptors (unused for now)

26879509f Add comments to descriptor tests (Pieter Wuille)
82df4c64f Add descriptor expansion cache (Pieter Wuille)
1eda33aab [refactor] Combine the ToString and ToPrivateString implementations (Pieter Wuille)
24d3a7b3a [refactor] Use DescriptorImpl internally, permitting access to new methods (Pieter Wuille)
6be0fb4b3 [refactor] Add a base DescriptorImpl with most common logic (Pieter Wuille)

Pull request description:

  This patch modifies the internal `Descriptor` class to optionally construct and use an "expansion cache". Such a cache is a byte array that encodes all information necessary to expand a `Descriptor` a second time without access to private keys, and without the need to perform expensive BIP32 derivations. For all currently defined descriptors, the cache simply contains a concatenation of all public keys used.

  This is motivated by the goal of importing a descriptor into the wallet and using it as a replacement for the keypool, where it would be impossible to expand descriptors if they use hardened derivation.

Tree-SHA512: f531a0a82ec1eecc30b78ba8a31724d1249826b028cc3543ad32372e1aedd537f137ab03dbffc222c5df444d5865ecd5cec754c1ae1d4989b6e9baeaffade32a

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