refactor: modernize the implementation of uint256.*

- 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
    - 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)
    - memcmp -> std::memcmp

ci/test/06_script_b.sh

@@ -57,6 +57,7 @@ if [ "${RUN_TIDY}" = "true" ]; then
           " src/rpc/signmessage.cpp"\
           " src/test/fuzz/txorphan.cpp"\
           " src/test/fuzz/util/"\
+          " src/uint256.cpp"\
           " src/util/bip32.cpp"\
           " src/util/bytevectorhash.cpp"\
           " src/util/check.cpp"\