Rewrite of IBM doublebyte charsets

[Ulf Zibis]

Sherman,
thanks for verifying my suggestions.

 > (1) simplify the "plane number" byte check by adding a new static 
array of cnspToIndex[16] for decoder
What you think about:

            int cnsPlane = sa[sp +1] ^ 0xa0;
            if (cnsPlane > 0xf ||
                           (cnsPlane = cnspToIndex[cnsPlane]) < 0)
                return CoderResult.malformedForLength(2);

or maybe (to force LoadUB, which may be faster than sign extension to int):
            int cnsPlane = (sa[sp +1] ^ 0xa0) && 0xff;

or maybe use byte[] (to force LoadUB, which may be faster than sign 
extension to int):
            byte cnsPlane = (byte)(sa[sp +1] ^ 0xa0);

or simply:

       static final byte[] cnspToIndex = new byte[0x100];
       static {
           Arrays.fill(cnspToIndex, -1);
           cnspToIndex[0xa2] = 1; cnspToIndex[0xa3] = 2; 
cnspToIndex[0xa4] = 3;
           cnspToIndex[0xa5] = 4; cnspToIndex[0xa6] = 5; 
cnspToIndex[0xa7] = 6; cnspToIndex[0xaf] = 7;
       }

            if ((cnsPlane = cnspToIndex[sa[sp + 1] && 0xff]) < 0)
                return CoderResult.malformedForLength(2);


*** Question: Why you code:

                   } else if ((byte1 & MSB) == 0) {  // ASCII  G0

instead of:

                   } else if (byte1 >= 0) {  // ASCII  G0

???

-Ulf