Merge bitcoin/bitcoin#26345: refactor: modernize the implementation of uint256.*

935acdcc79 refactor: modernize the implementation of uint256.* (pasta)

Pull request description:

  - Constructors of uint256 to utilize Span instead of requiring a std::vector
  - converts m_data into a std::array
  - Prefers using `WIDTH` instead of `sizeof(m_data)`
  - make all the things constexpr
  - replace C style functions with c++ equivalents
      - memset -> std::fill
          This may also be replaced by std::memset, but I think that std::fill is more idiomatic of modern c++ and readable.
      - memcpy -> std::copy
          Note: In practice, implementations of std::copy avoid multiple assignments and use bulk copy functions such as std::memmove if the value type is TriviallyCopyable and the iterator types satisfy LegacyContiguousIterator. (https://en.cppreference.com/w/cpp/algorithm/copy)
          This could also likely be replaced by std::memcpy, but as said above, I believe the using std::copy is the more c++ way to do anything and is almost guaranteed to compile to the same asm
      - memcmp -> std::memcmp

ACKs for top commit:
  achow101:
    ACK 935acdcc79
  hebasto:
    Approach ACK 935acdcc79.
  aureleoules:
    reACK 935acdcc79
  john-moffett:
    ACK 935acdcc79
  stickies-v:
    Approach ACK 935acdcc7

Tree-SHA512: 4f1ba54ff2198eea0e505d41e73d552c84c60f6878d5c85a94a8ab57f39afc94ef8d79258e7afd01fa84ec2a99f4404bb877eecd671f65e1ee9273f3129fc650

src/psbt.h

@@ -889,7 +889,7 @@ struct PSBTOutput
                     } else if (key.size() != 33) {
                         throw std::ios_base::failure("Output Taproot BIP32 keypath key is not at 33 bytes");
                     }
-                    XOnlyPubKey xonly(uint256({key.begin() + 1, key.begin() + 33}));
+                    XOnlyPubKey xonly(uint256(Span<uint8_t>(key).last(32)));
                     std::set<uint256> leaf_hashes;
                     uint64_t value_len = ReadCompactSize(s);
                     size_t before_hashes = s.size();