* calculate MainPod id in a dynamic-friendly way
The MainPod id is now calculated with front padding and a fixed size
independent of max_public_statements so that introduction gadgets can be
verified by a MainPod while paying only for the number of statements
they use. This is because with front padding of none-statements we can
precompute the poseidon state corresponding to absorbing all the padding
statements and only pay constraints for the non-padding statements.
The id is calculated as follows:
`id = hash(serialize(reverse(statements || none-statements)))`
* add time feature and disable timing by default
* apply suggestions from @arnaucube
* link issues in todos
