Change hash of reveal input data to 32 bytes

Currently reveal input requires a Sc_rollup_PVM_sig.Input_hash.t which has a size of 20 bytes. There is a consensus that 20 bytes is not cryptographically secure towards collision attacks. Furthermore, the Tx Kernel host function expects a hash of 32 bytes. The Reveal input should be changed to accept a new hash of hash, which has a fixed size of 32 bytes.