quant_4x4x4: quant one 8x8 block at a time

This reduces overhead and lets us use less branchy code for zigzag, dequant,
decimate, and so on.
~17% faster macroblock_encode with 4x4dct.
Includes NEON and x86 versions of the new function.

Using larger merged functions like this will also make wider SIMD, like
AVX2, more effective.