Add simpler version of update_cdf() for boolean symbols
Average decoding time of first 1k frames of Chimera 8bit 1080p goes down in my tests from 14.807 seconds to 14.765 seconds (average of 6 runs). Might also make life easier for Rostislav with his msac_update_cdf()
SIMD work.
Edited by Ronald S. Bultje