v2.1.6: Encode and decode speed improvements

1. In td512.c, modified checkTextMode to return a code that causes use of extended string mode and improves compression speed. Also added ' (quote) to the text chars array.
2. In td512.c, modified checktd64 (name change from checkSingleValueMode) to return a code for expected random data to be processed as a failing block of 64 bytes in the same way that td64 would handle this block.
3. In td64.c, modified decodeAdaptiveTextMode to read one byte ahead to improve speed of processing dtbmPeekBits. This change can require one byte read beyond length of input array.