Use 4 byte lookups instead of 8 in `SyscallReadPreimage`

[querolita]

See #2277

The idea is to use only 4 lookups in each SyscallReadPreimage instruction instead of 8. Meaning it just looks up the at most 4 bytes read at each call, but is able to tell which ones represent length and which ones represent preimage, so that only the preimage bytes are used for the communication channel.