Speed and memory usage improvements for the decoder.

* Change order of members of bit reader state structure.

* Remove unused includes for assert. Add BROTLI_DCHECK
  macros and use it instead of assert.

* Do not calculate nbits in common case of ReadSymbol.

* Introduce and use PREDICT_TRUE / PREDICT_FALSE macros.

* Allocate less memory in the brotli decoder if it knows
  the result size beforehand. Before this, the decoder
  would always allocate 16MB if the encoder annotated the
  window size as 22 bit (which is the default), even if the
  file is only a few KB uncompressed. Now, it'll only
  allocate a ringbuffer as large as needed for the result file.
  But only if it can know the filesize, it's not possible
  to know that if there are multiple metablocks or too large
  uncompressed metablock.