indexing for EAV

[Connoropolous]

@thedavidmeister said...

it's just adding a local index for each stored EAV to EAVI so we can get the "current" value for any given EA

if the data is added on the way in then IIRC it should be easy enough to get the right value with the existing iterators

long term it might be worth adding a new method to the eav storage trait so that implementations can optimise this lookup, e.g. with indexes and sorting on the backend

but for now why not just stick this logic in rust?