Structured Append missing "N"

[kousu]

The "structured append" paging header has three parts: M, N and parity. M is the index of the current code, N is the number of codes in the set, and parity is the checksum to identify them all as belonging together. M and N are both 4 bits, but

https://github.com/nu-book/zxing-cpp/blob/e1d7de9cba1d912a3a13bd5e4c2e6deb80643d41/core/src/qrcode/QRDecoder.cpp#L340-L348

pulls both M and N into the same 8 bit variable. They should be separated and exposed as separate values. The DecoderResult misleadingly calls this 8-bit (M,N) tuple the "sequence number":

https://github.com/nu-book/zxing-cpp/blob/e1d7de9cba1d912a3a13bd5e4c2e6deb80643d41/core/src/qrcode/QRDecoder.cpp#L414

when in fact only the first four bits are that.