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.
CABAC/CAVLC: use the new bit-iterating macro here too
Significantly faster.
Add slice-min-mbs feature

Works in conjunction with slice-max-mbs and/or slice-max-size to avoid overly
small slices.
Useful with certain decoders that barf on extremely small slices.

If slice-min-mbs would be violated as a result of slice-max-size, x264 will
exceed slice-max-size and print a warning.
Add slices-max feature

Certain decoders may break with extremely large numbers of slices, as can
occur with some slice-max-size/mbs settings.

When set, x264 will refuse to create any slices beyond the maximum number,
even if slice-max-size/mbs requires otherwise.

Experimental: using gcc atomics to implement this with sliced threads?
CABAC residual asm: draft of checkasm