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.
Reorganize and optimize a lot of macroblock_encode using this new function.
~1-2% faster overall.

Includes NEON and x86 versions of the new function.
Using larger merged functions like this will also make wider SIMD, like
AVX2, more effective.
x86_64 cabac_block_residual assembly

RDO: ~20% faster than C
Bitstream: ~50% faster than C
1-2% faster overall, highest on preset superfast/fast/medium.