Interop: update the event-index wildcard / intent feature PR

[protolambda]

The idea of specifying a wildcard event-index, to defer the exact connection to initiating-event to the sequencer, was drafted here: #108

This needs to be updated to account for executing-messages now being declared through events, not statically in a transaction.
Likewise, the link to the initiating message, as inserted by the sequencer, may be declared through an event, to keep all message-dependency data within the log-events scope.

This enables more flexible transaction flows, by allowing users to pre-sign multiple interdependent interop txs, without pre-committing to the exact block construction (i.e. no exact event index).