Assert that ForUtil.BLOCK_SIZE can be encoded in a single byte in PForUtil

PForUtil assumes that ForUtil.BLOCK_SIZE can be encoded in a single byte when generating "patch offsets". If this assumption doesn't hold, PForUtil will silently encode incorrect positions. While the BLOCK_SIZE isn't particularly configurable, it would be nice to assert this assumption early in PForUtil in the even that the BLOCK_SIZE changes in some future codec version.