Add MEMPOOL rule

[lehins]

This MEMPOOL rule will be called by the LEDGER rule, regardless of the isValid flag.

It will only be called if the LedgerEnv will have a new flag leForMempool :: Bool set to True

This will rule will have a generic predicate failure:

newtype MempoolPredFailure era = MempoolPredFailure Text

that has a free form description of a failure.

Most of the time this rule will be empty, but because it will only be invoked on transactions that are being placed into the mempool it does not require a hardfork to have its behavior amended at any point in time.

In other words when LEDGERS rule will invoke LEDGER then leForMempool will be set to False and the MEMPOOL will not be invoked, thus preventing any change in Ledger behavior that could be induced by this new rule.

We will thus be able to use this rule as a sanitizer of undesired transactions with any node release without requiring a hard fork