Cardano Updates

Implements `genSnapShots` in `Test.Cardano.Ledger.Conway.Gen.SnapShots`,
which generates a plausible `SnapShots` value from a `LedgerState` for
use in tests.

The base snapshot is derived from the current instant stake via
`snapShotFromInstantStake`. Mark, set, and go snapshots are then
produced by altering the base snapshot which randomly samples zero or
more of the following operations and applies them in sequence:

- New stake registration: introduce a fresh credential delegated to an existing pool
- Stake address deregistration: remove a credential from active stake
- Stake address redelegation: redirect an already-delegated credential to a different pool
- First delegation: delegate a registered-but-undelegated credential to a pool
- Stake pool registration: add a new pool
- Stake amount change: adjust the stake of an existing credential
- Pool deregistration (set/go only): remove a pool and all its delegators

Pool deregistration is excluded from the mark snapshot because the SNAP
rule (which records the current stake distribution) runs before POOLREAP
(which processes pool retirements that have reached their epoch) in the
epoch transition, so retiring pools are still visible in mark.

Starting from def dropped enacted, expired and delayed actions, losing
the removal, deposit refund and event for actions enacting at the era
boundary.

Add Test.Cardano.Ledger.Conway.Gen.{UTxO,Script} modules.

Add the generator for valid scripts.

Provide generator for conway-based UTxO with valid domain values.

by extracting the shared logic in helpers

so it can be reused in Alonzo

in `mkStAnnTx`

for more per-era control over revalidation of transactions

to replace `applyTxValidation`, operating on `ValidatedTx`.
Define `applyTxValidation` in terms of `applyTxWithValidation`