Optimize read-write memory commitments

[moodlezoup]

Avoids a couple of commitments and constraints by being smarter about how we compute (a, v, t) tuples for read-write memory, namely:

• rs1 and rs2 are read-only, so we know v_write = v_read and t_write = global_timestamp
• rd is write-only, so we know t_write = global_timestamp + 1
• The four bytes of RAM are contiguous, so we only need to commit to the address of the first byte