Further improve the way the near-optimal parser estimates symbol costs:
- When setting a block's initial costs, weigh the default costs and
previous block's costs differently, depending on how different the
current block seems to be from the previous block.
- When determining the "default" costs, take into account how many
literals appear in the block and how frequent matches seem to be.
- Increase BIT_COST from 8 to 16, to increase precision in calculations.
