ec_smallmul: Simplify binary read/write.

This should be the same number of operations as the non-ec_smallmul
version (though ideally we'd use the real 15-bit probability
natively).

Encoder output should not change, and all streams should remain
decodable without decoder changes.

Change-Id: I2998a77a02f566cd0c82c415395637acf49b5a97
2 files changed