codec/h264/h264dec: move cabac 'binarization' stuff to enc.go file in cabac pkg

The binarization processes are slightly wrong in that they are providing a bin string for a particular syntax value, rather than providing a set of bin strings for all possible syntax values. This is more useful for cabac encoding, so it is proposed that this code be moved to a file called enc.go. It’s also proposed that this file and the current file cabac.go be moved to a package called cabac.