Move fromhexstr() to str_util

src/util/str_util.h

@@ -37,5 +37,6 @@ std::string format_datetime(uint64_t unixtime);
 bool is_zero(void *buf, size_t size);
 bool memcheck(uint8_t *buf, uint8_t byte, size_t len);
 std::string urldecode(const std::string & orig);
+size_t fromhexstr(const std::string & from, size_t bytes, uint8_t *to);
 
 #pragma GCC visibility pop