Improved event syscall API

[Stebalien]

Currently, events are emitted as CBOR objects. Unfortunately, this makes it hard to accurately charge gas for parsing.

Proposal:

Emit three buffers:

• entries: An array of [(flags: u64, codec: u64, key_len: u32, value_len: u32), ...] (packed). I.e., 24 bytes per event.
• keys: A buffer of all keys, concatenated.
• values: A buffer of all values, concatenated.

The FVM will:

1. Compute the number of entries as entries.len() / 24 (or return an error if not a multiple of 24).
2. Charge gas based on the number of entries, the length of keys, and the length of values.
3. Reconstruct the events from these parameters.

Importantly, this means the FVM can accurately charge gas before parsing anything.

[aashidham]

Does this have any product implications with the emission of events? What about event listeners?

Will this generally make sending events to the event log more or less expensive?

[Stebalien]

This won't have any user-facing implications.

This issue is purely security/performance related. Although it may make emitting events less expensive as it may be less expensive to validate them? But that's unclear.