Interface to fold instances when reaching 2^15 entries

[joseandro]

• Right now, in the main.rs, everytime the step_index (keccak) or instruction_counter (mips) hits 2^15, the code calls the fold function.
• This assumes we just have 2 instances: one for keccak, another one for mips.
• If we want to have multiple instances for each subtype, then we will need to have counters for each of them. Same for the lookups.