! @param[in] len the length of the object or static_cast<std::size_t>(-1) for an object of indefinite size @param[in] tag

! @brief reads a CBOR string This function first reads starting bytes to determine the expected string length and then copies this number

! @brief get codepoint from 4 hex characters following `\u` For input "\u c1 c2 c3 c4" the codepoint is: (c1 * 0x1000) + (c2 * 0x0100)