[Feature Request][compiler-v2] Overload compound operator to apply to aggregators

[alnoki]

@georgemitenkov @wrwg

• Allow aggregator::add(my_agg, amount) to be represented as my_agg += amount

Note per @wrwg in Move Interest Group Telegram chat:

We like to wait with this until we have traits, which are currently in development. With traits we'd have trait AssignAdd { fun assign_add(self: &mut Self, other: &Self) } which can be defined to enable += for aggregators and other types.