Includes unit tests for conformance and speed.

SSSE2/CFLSubAvgTest
4x4: C time = 234 us, SIMD time = 152 us (~1.5x)
8x8: C time = 664 us, SIMD time = 208 us (~3.2x)
16x16: C time = 1687 us, SIMD time = 581 us (~2.9x)
32x32: C time = 6118 us, SIMD time = 2119 us (~2.9x)

AVX2/CFLSubAvgTest
4x4: C time = 250 us, SIMD time = 221 us (~1.1x)
8x8: C time = 683 us, SIMD time = 284 us (~2.4x)
16x16: C time = 1727 us, SIMD time = 1091 us (~1.6x)
32x32: C time = 6092 us, SIMD time = 2107 us (~2.9x)

Change-Id: I44ffedc683829d2c16089854ac43d4ddb4415bcd