Partial butterfly functions now utilize the strategy selector
Added AVX2 strategy for partial butterfly (no optimizations yet)
Partial butterfly strategy is now called DCT strategy. Made changes to transform functions in preparation for optimizations.

-Moved fast_forward_dst and fast_inverse_dst to DCT strategies
Add benchmark test for dct functions.
Removed all AVX2 instructions from SATD functions.

-Zero extend macro now returns results in 2 xmm registers instead of one ymm