Improve Event Syscall API

[Stebalien]

Currently, events are emitted as CBOR objects. Unfortunately, this makes it hard to accurately charge gas for parsing. We currently handle this by tuning our gas model for EVM events but that's obviously not going to cut it.

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.

Implementation:

This has already been implemented in https://github.com/filecoin-project/ref-fvm/pull/1807/files and merged into a "next" branch in the ref-fvm (it's consensus breaking so we don't want it in master).

[anorth]

Sounds fine to me.