Simplification of Arith_codec to remove unnecessary
variables and make the code clearer. Merging together
the DecodeSymbol / RemFromStream clarifies the
algorithm and allows some optimisation.
Change how contexts are chosen in BandContext to
reduce number of calls to ChooseContext.
Decoding at quality factor 7 gives a reduction of 2.5% at
CIF and 10% at HD720.