fix: add event index to transaction events

[janniks]

• closes return event_index in Stacks transaction event metadata #387
• adds a position struct to the event and renames the event to payload (with enum type)
• updates occurences.json to include the expected serialized position for testing

[MicaiahReid]

Thanks, @janniks!!

[MicaiahReid]

I've discovered some ways where this may cause breakages, I need to do some investigation.

[MicaiahReid]

Okay, it looks like it should be good!

[lgalabru]

Thanks @janniks! We already have the struc StacksTransactionPosition in place, indicating the position of a transaction in the block or in the microblock, could we use this instead?
Apologies for the latency on this review.

[janniks]

Oh yeah sure, sorry for missing that 👍

[janniks]

The new positions are referring to events not blocks. Should we create an enum type that mixes blocks and events (might be funky when matching). Or add an additional enum layer EventPosition (but that also sounds complected)?

[MicaiahReid]

@lgalabru I don't know if I agree - Jannik's change is adding a struct for an event's position in a transaction, while StacksTransactionPosition represents a transaction's position within a block.

I guess we could abstract that position enum to be more general, from:

pub enum StacksTransactionPosition {
    AnchorBlock(AnchorBlockPosition),
    MicroBlock(MicroBlockPosition),
}

to:

pub enum Position {
    TransactionPositionInAnchorBlock(AnchorBlockPosition),
    TransactionPositionInMicroBlock(MicroBlockPosition),
    EventPositionInTransaction(u64)
}

but that feel's clunky. Let me know if we're misunderstanding what you meant here!

[lgalabru]

My apologies for the latency on this PR, yes that makes sense to me.
Thank you for your help @janniks!

[MicaiahReid]

Closing in favor of #495