935acdc 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 935acdc
  hebasto:
    Approach ACK 935acdc.
  aureleoules:
    reACK 935acdc
  john-moffett:
    ACK 935acdc
  stickies-v:
    Approach ACK 935acdc

Tree-SHA512: 4f1ba54ff2198eea0e505d41e73d552c84c60f6878d5c85a94a8ab57f39afc94ef8d79258e7afd01fa84ec2a99f4404bb877eecd671f65e1ee9273f3129fc650