Implements a state machine for contracts and their keys while interpreting a Daml-LF command or iterating over a com.daml.lf.transaction.HasTxNodes in execution order. The contract state machine keeps track of the updates to the ContractStateMachine.ActiveLedgerState since the beginning of the interpretation or iteration. Given a ContractStateMachine.State s, a client can compute the next state for a given action node n, by calling s.handleNode(..., n, ...). For a rollback node nr, a client must call beginRollback before processing the first child of nr and endRollback after processing the last child of nr.