Accept generics in `AggregateState` for aggregate id

[cottinisimone]

The idea is based on possibly having a NewType for the aggregate id, so be more flexible in handling those types.

Possible solutions could be:

• Add a new generic type to AggregateState with consequent associated type in Aggregate to have a T (like AggregateId type) implementing Into<Uuid> and From<Uuid>.
• Modify AggregateState::new to get an impl Into<Uuid>

[cottinisimone]

I propose to put this when rust-lang/rfcs#2532 will be included in the stdlib. The issue: rust-lang/rust#29661

[mystique09]

Hi, any status on this? The referenced rust issue has been merged.