add event/log recording and reporting machinery for contracts


- [x] Make event-log buffer in host
- [x] Make host functions and host infrastructure for logging diagnostic / error messages
- [x] Define general / non-diagnostic event type in XDR-next
- [x] Emit relevant host events into event-log buffer
- [x] Route events back to transaction-invoker / preflight-invoker
- [x] Emit events into the meta stream