use lookup tables instead of actual exp/pow for AQ

use lookup tables instead of actual exp/pow for AQ
Significant speed boost, especially on CPUs with atrociously slow floating point units (e.g. Pentium 4 saves 800 clocks per MB with this change).
Add x264_clz function as part of the LUT system: this may be useful later.
Note this changes output somewhat as the numbers from the lookup table are not exact.