Proto/Michelson: event logging

Context

This MR aims to provide MVP support for event logging, as planned in this TZIP MR. The rendered version of the TZIP is here.

Event logging provides means to emit event-like data for external consumption, which is attached to the transaction receipt. The MVP will enable applications like DApps, cross-chain services and bridges, and event consumers and indexers in general, to respond to changes to on-chain states or transaction-to-contract processes.

There are still rough edges around this work. Here are some pending items to resolve.

• Details in the proposal
• Verification of correct event emission
• Access to emitted events from tezos-client

Manually testing the MR

So far, the easiest access to this functionality is through simulation with tezos-client on a Michelson contract with EMIT instruction. Examples are provided in both the RFC and the unit test included in this MR.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR