intx 0.15.0

Added

• Add bit_width() helper. #356
• Add load() and store() helpers with std::span<uint8_t> parameters. #366
• Add support for 0X prefix to from_string(). #361
• Add constructor from builtin_uint128 to avoid truncation. #355
• Add constructor from std::span<uint64_t>. #362

Changed

• Git default branch changed to main.
• Optimize exp() by traversing exponent bits from the top. #357
• Change as_words() to return fixed-size std::span<uint64_t>. #364
• Add explicit implementations of assignment operators. #353
• Use std::span for division internals and helper procedures. #365