lossy bit-reader clean-up:

* remove LEFT/RIGHT_JUSTIFY distinction. It's all RIGHT_JUSTIFY now.
* simplify VP8GetSigned(), and add some masking branch-less code. Much
  faster on ARM (~13% speed-up). 8% on x86-64, 5% on MacBook.
* split critical implementation into separate bit_reader_inl.h file that
  is only included where needed (vp8.c / tree.c / bit_reader.c)
* bumped BITS value from 16 to 24 for x86-32b too, since it's a bit faster.

Change-Id: If41ca1da3e5c3dadacf2379d1ba419b151e7fce8
Merge "lossy bit-reader clean-up:"
remove libwebpdspdecode dep on libwebpdsp_avx2

it's encode only, libwebpdecoder doesn't need the symbols

Change-Id: I5633dd2017a96e60068ae5384f1ba27898d29f83
Merge "remove libwebpdspdecode dep on libwebpdsp_avx2"
restore original value_/range_ field order

no speed change, just for coherency

Change-Id: Iaa395bca24f33a14b68ba6920b838ef87d0d0db6