Add field names to pallet `Event` variants

[ascjones]

As per #9896, we can convert other pallets to use struct variants with named fields.

e.g. Instantiated(T::AccountId, T::AccountId) becomes Instantiated { deployer: T::AccountId, contract: T::AccountId }.

These field names and their respective comments are in turn reflected in the metadata for a better client experience.

[Wizdave97]

PIcking this up