Move fromhexstr() to str_util

src/util/str_util.h

@@ -35,5 +35,6 @@ std::string realpath_str(std::string path, bool nofail = true);
 std::string format_datetime(uint64_t unixtime);
 bool is_zero(void *buf, size_t size);
 std::string urldecode(const std::string & orig);
+size_t fromhexstr(const std::string & from, size_t bytes, uint8_t *to);
 
 #pragma GCC visibility pop