DataBus: robust handling of read_counts

[ledwards2225]

Currently calldata_read_counts must be manually updated simultaneously with each calldata lookup gate. This is error prone. The polynomial calldata_read_counts should be computed all at once either in finalize or in the composer. One way to do this: Iterate through all databus lookup gates (looking for q_busread == 1), extract the read_index (stored in w_r), call get_variable on that index, convert the resulting field element to a size_t or uint32_t, then increment calldata_read_counts at the resulting index.