Move non-trivial uint256.h methods to uint256.cpp

src/util.h

@@ -156,6 +156,7 @@ bool ParseMoney(const char* pszIn, int64_t& nRet);
 std::string SanitizeString(const std::string& str);
 std::vector<unsigned char> ParseHex(const char* psz);
 std::vector<unsigned char> ParseHex(const std::string& str);
+signed char HexDigit(char c);
 bool IsHex(const std::string& str);
 std::vector<unsigned char> DecodeBase64(const char* p, bool* pfInvalid = NULL);
 std::string DecodeBase64(const std::string& str);